error during running a code to generate a view flutter

cantieri_screen_view.dart

import 'dart:async';

import 'package:MyApp/Controller/Cantiere.dart';
import 'package:MyApp/Model/Utente.dart';
import 'package:MyApp/View/Cantieri/scheda_cantiere_view.dart'
    show SchedaCantieriView;
import 'package:flutter/material.dart';
import 'package:MyApp/Model/Cantiere.dart';
import 'package:MyApp/utils/support.dart';
import '../Home_theme.dart';

//Variabile che rappresenta l'utente all'interno della view
Utente utemp;
//Questa map rappresenta i cantieri ricercati
List<Cantiere> cantieriricercati;
//La funzione di inizializzazione esegue l'init di tutti i valori che
//verranno utilizzati nella view
Future<void> inizializzaValori() async {
  //Estrapolo i valori per inizializzare l'utente
  var email = await Storage.leggi("Email");
  var password = await Storage.leggi("Password");
  var idutente = int.parse(await Storage.leggi("IdUtente"));
  //Inizializzo l'utente
  utemp = new Utente.init(idutente, email, password);
  //Inizializzo i cantieri all'avvio della form
  cantieriricercati = await Cantiere.ricerca(utemp, 0, "", "", false);
  //Stampa dei valori dei cantieri ricercati
  for (int i = 0; i < cantieriricercati.length; i++) {
    print("Cantieri ricercati -- Nome Cantiere:  " +
        cantieriricercati[i].getNomeCantiere());
  }
}

class TrainingScreen extends StatefulWidget {
  const TrainingScreen({Key key, this.animationController}) : super(key: key);

  final AnimationController animationController;
  @override
  _TrainingScreenState createState() => _TrainingScreenState();
}

class _TrainingScreenState extends State<TrainingScreen>
    with TickerProviderStateMixin {
  Animation<double> topBarAnimation;

  List<Widget> listViews = <Widget>[];
  final ScrollController scrollController = ScrollController();
  double topBarOpacity = 0.0;

  @override
  void initState() {
    topBarAnimation = Tween<double>(begin: 0.0, end: 1.0).animate(
        CurvedAnimation(
            parent: widget.animationController,
            curve: Interval(0, 0.5, curve: Curves.fastOutSlowIn)));
    addAllListData();

    scrollController.addListener(() {
      if (scrollController.offset >= 24) {
        if (topBarOpacity != 1.0) {
          setState(() {
            topBarOpacity = 1.0;
          });
        }
      } else if (scrollController.offset <= 24 &&
          scrollController.offset >= 0) {
        if (topBarOpacity != scrollController.offset / 24) {
          setState(() {
            topBarOpacity = scrollController.offset / 24;
          });
        }
      } else if (scrollController.offset <= 0) {
        if (topBarOpacity != 0.0) {
          setState(() {
            topBarOpacity = 0.0;
          });
        }
      }
    });
    super.initState();
  }

  void addAllListData() {
    const int count = 5;

    listViews.add(
      new TextFormField(
        decoration: new InputDecoration(
          labelText: "Ricerca Cantieri",
          fillColor: Colors.white,
          border: new OutlineInputBorder(
            borderRadius: new BorderRadius.circular(25.0),
            borderSide: new BorderSide(),
          ),
          //fillColor: Colors.green
        ),
        validator: (val) {
          if (val.length == 0) {
            return "Il campo non può essere vuoto";
          } else {
            return null;
          }
        },
        keyboardType: TextInputType.emailAddress,
        style: new TextStyle(
          fontFamily: "WorkSansSemiBold",
        ),
      ),
    );

//Aggiunta alla list della view corrispondente al cantiere

    listViews.add(
      SchedaCantieriView(
        animation: Tween<double>(begin: 0.0, end: 1.0).animate(CurvedAnimation(
            parent: widget.animationController,
            curve:
                Interval((1 / count) * 2, 1.0, curve: Curves.fastOutSlowIn))),
        animationController: widget.animationController,
      ),
    );

    //Eseguo la ricerca dei cantieri
    for (int i = 0; i < cantieriricercati.length; i++) {
      //Aggiunta alla list della view corrispondente al cantiere
      listViews.add(
        SchedaCantieriView(
          animation: Tween<double>(begin: 0.0, end: 1.0).animate(
              CurvedAnimation(
                  parent: widget.animationController,
                  curve: Interval((1 / count) * 2, 1.0,
                      curve: Curves.fastOutSlowIn))),
          animationController: widget.animationController,
          ctemp: cantieriricercati[i],
        ),
      );
    }
  }

  Future<bool> getData() async {
    await Future<dynamic>.delayed(const Duration(milliseconds: 50));
    return true;
  }

  @override
  Widget build(BuildContext context) {
    return Container(
      color: FintnessAppTheme.background,
      child: Scaffold(
        backgroundColor: Colors.transparent,
        body: FutureBuilder(
          future: inizializzaValori(),
          builder: (BuildContext context, AsyncSnapshot snapshot) {
            Stack(
              children: <Widget>[
                getMainListViewUI(),
                getAppBarUI(),
                SizedBox(
                  height: MediaQuery.of(context).padding.bottom,
                )
              ],
            );
          },
        ),
      ),
    );
  }

  Widget getMainListViewUI() {
    return FutureBuilder<bool>(
      future: getData(),
      builder: (BuildContext context, AsyncSnapshot<bool> snapshot) {
        if (!snapshot.hasData) {
          return const SizedBox();
        } else {
          return ListView.builder(
            controller: scrollController,
            padding: EdgeInsets.only(
              top: AppBar().preferredSize.height +
                  MediaQuery.of(context).padding.top +
                  24,
              bottom: 62 + MediaQuery.of(context).padding.bottom,
            ),
            itemCount: listViews.length,
            scrollDirection: Axis.vertical,
            itemBuilder: (BuildContext context, int index) {
              widget.animationController.forward();
              return listViews[index];
            },
          );
        }
      },
    );
  }

  Widget getAppBarUI() {
    return Column(
      children: <Widget>[
        AnimatedBuilder(
          animation: widget.animationController,
          builder: (BuildContext context, Widget child) {
            return FadeTransition(
              opacity: topBarAnimation,
              child: Transform(
                transform: Matrix4.translationValues(
                    0.0, 30 * (1.0 - topBarAnimation.value), 0.0),
                child: Container(
                  decoration: BoxDecoration(
                    color: FintnessAppTheme.white.withOpacity(topBarOpacity),
                    borderRadius: const BorderRadius.only(
                      bottomLeft: Radius.circular(32.0),
                    ),
                    boxShadow: <BoxShadow>[
                      BoxShadow(
                          color: FintnessAppTheme.grey
                              .withOpacity(0.4 * topBarOpacity),
                          offset: const Offset(1.1, 1.1),
                          blurRadius: 10.0),
                    ],
                  ),
                  child: Column(
                    children: <Widget>[
                      SizedBox(
                        height: MediaQuery.of(context).padding.top,
                      ),
                      Padding(
                        padding: EdgeInsets.only(
                            left: 16,
                            right: 16,
                            top: 16 - 8.0 * topBarOpacity,
                            bottom: 12 - 8.0 * topBarOpacity),
                        child: Row(
                          mainAxisAlignment: MainAxisAlignment.center,
                          children: <Widget>[
                            Expanded(
                              child: Padding(
                                padding: const EdgeInsets.all(8.0),
                                child: Text(
                                  'Cantieri',
                                  textAlign: TextAlign.left,
                                  style: TextStyle(
                                    fontFamily: FintnessAppTheme.fontName,
                                    fontWeight: FontWeight.w700,
                                    fontSize: 22 + 6 - 6 * topBarOpacity,
                                    letterSpacing: 1.2,
                                    color: FintnessAppTheme.darkerText,
                                  ),
                                ),
                              ),
                            ),

                            /*SizedBox(
                              height: 38,
                              width: 38,
                              child: InkWell(
                                highlightColor: Colors.transparent,
                                borderRadius: const BorderRadius.all(
                                    Radius.circular(32.0)),
                                onTap: () {},
                                child: Center(
                                  child: Icon(
                                    Icons.keyboard_arrow_right,
                                    color: FintnessAppTheme.grey,
                                  ),
                                ),
                              ),
                            ),*/
                          ],
                        ),
                      )
                    ],
                  ),
                ),
              ),
            );
          },
        )
      ],
    );
  }
}

Error

Failed assertion: line 1651 pos 12: '!_debugDoingThisLayout': is not true. flutter: Another exception was thrown: NoSuchMethodError: The getter 'length' was called on null. flutter: Another exception was thrown: Each child must be laid out exactly once. flutter: Another exception was thrown: NoSuchMethodError: The getter 'length' was called on null. flutter: Another exception was thrown: Each child must be laid out exactly once. [VERBOSE-2:ui_dart_state.cc(157)] Unhandled Exception: Cannot hit test a render box that has never been laid out. The hitTest() method was called on this RenderBox: RenderStack#8e720 NEEDS-LAYOUT NEEDS-PAINT: creator: Stack ← FutureBuilder ← _BodyBuilder ← MediaQuery ← LayoutId-[<_ScaffoldSlot.body>] ← CustomMultiChildLayout ← AnimatedBuilder ← DefaultTextStyle ← AnimatedDefaultTextStyle ← _InkFeatures-[GlobalKey#19263 ink renderer] ← NotificationListener ← PhysicalModel ← ⋯ parentData: offset=Offset(0.0, 0.0); id=_ScaffoldSlot.body constraints: MISSING size: MISSING alignment: AlignmentDirectional.topStart textDirection: ltr fit: loose overflow: clip Unfortunately, this object's geometry is not known at this time, probably because it has never been laid out. This means it cannot be accurately hit-tested. If you are trying to perform a hit test during the layout phase itself, make sure you only hit test nodes that have complet<…>

scheda_cantiere_view.dart

import 'package:flutter/material.dart' ;
import 'package:MyApp/Model/Cantiere.dart';
import '../Home_theme.dart';
import 'package:MyApp/Utils/Style/color.dart';


//Classe che rappresenta la scheda di un cantiere
class SchedaCantieriView extends StatelessWidget {
  final AnimationController animationController;
  final Animation animation;
  //Oggetto che rappresenta il cantiere 
  final Cantiere ctemp;

  //Costruttore
  const SchedaCantieriView({Key key, this.animationController, this.animation, this.ctemp})
      : super(key: key);

  @override
  Widget build(BuildContext context) {
    return AnimatedBuilder(
      animation: animationController,
      builder: (BuildContext context, Widget child) {
        return FadeTransition(
          opacity: animation,
          child: new Transform(
            transform: new Matrix4.translationValues(
                0.0, 30 * (1.0 - animation.value), 0.0),
            child: Padding(
              padding: const EdgeInsets.only(
                  left: 24, right: 24, top: 16, bottom: 18),
              child: Container(
                decoration: BoxDecoration(
                  gradient: LinearGradient(colors: [
                    FintnessAppTheme.nearlyDarkBlue,
                    HexColor("#6F56E8")
                  ], begin: Alignment.topLeft, end: Alignment.bottomRight),
                  borderRadius: BorderRadius.only(
                      topLeft: Radius.circular(8.0),
                      bottomLeft: Radius.circular(8.0),
                      bottomRight: Radius.circular(8.0),
                      topRight: Radius.circular(68.0)),
                  boxShadow: <BoxShadow>[
                    BoxShadow(
                        color: FintnessAppTheme.grey.withOpacity(0.6),
                        offset: Offset(1.1, 1.1),
                        blurRadius: 10.0),
                  ],
                ),
                child: Padding(
                  padding: const EdgeInsets.all(16.0),
                  child: Column(
                    mainAxisAlignment: MainAxisAlignment.center,
                    crossAxisAlignment: CrossAxisAlignment.start,
                    children: <Widget>[
                      Text(
                        '',
                        textAlign: TextAlign.left,
                        style: TextStyle(
                          fontFamily: FintnessAppTheme.fontName,
                          fontWeight: FontWeight.normal,
                          fontSize: 14,
                          letterSpacing: 0.0,
                          color: FintnessAppTheme.white,
                        ),
                      ),
                      Padding(
                        padding: const EdgeInsets.only(top: 8.0),
                        child: Text(
                          'Cantiere: '+ctemp.getNomeCantiere().toString()+' \n\nCliente: '+ctemp.getRagioneSociale()+'',
                          textAlign: TextAlign.left,
                          style: TextStyle(
                            fontFamily: FintnessAppTheme.fontName,
                            fontWeight: FontWeight.normal,
                            fontSize: 20,
                            letterSpacing: 0.0,
                            color: FintnessAppTheme.white,
                          ),
                        ),
                      ),
                      SizedBox(
                        height: 32,
                      ),
                      Padding(
                        padding: const EdgeInsets.only(right: 4),
                        child: Row(
                          crossAxisAlignment: CrossAxisAlignment.end,
                          mainAxisAlignment: MainAxisAlignment.center,
                          children: <Widget>[
                            Padding(
                              padding: const EdgeInsets.only(left: 4),
                              child: Icon(
                                Icons.timer,
                                color: FintnessAppTheme.white,
                                size: 16,
                              ),
                            ),
                            Padding(
                              padding: const EdgeInsets.only(left: 4.0),
                              child: Text(
                                ''+ctemp.getDataCreazione(),
                                textAlign: TextAlign.center,
                                style: TextStyle(
                                  fontFamily: FintnessAppTheme.fontName,
                                  fontWeight: FontWeight.w500,
                                  fontSize: 14,
                                  letterSpacing: 0.0,
                                  color: FintnessAppTheme.white,
                                ),
                              ),
                            ),
                            Expanded(
                              child: SizedBox(),
                            ),
                            Container(
                              decoration: BoxDecoration(
                                color: FintnessAppTheme.nearlyWhite,
                                shape: BoxShape.circle,
                                boxShadow: <BoxShadow>[
                                  BoxShadow(
                                      color: FintnessAppTheme.nearlyBlack
                                          .withOpacity(0.4),
                                      offset: Offset(8.0, 8.0),
                                      blurRadius: 8.0),
                                ],
                              ),

                              //Bottone che permette l'accesso al cantiere
                              child: Padding(
                                padding: const EdgeInsets.all(0.0),
                                child: Icon(
                                  Icons.arrow_right,
                                  color: HexColor("#6F56E8"),
                                  size: 44,
                                ),
                              ),
                            )
                          ],
                        ),
                      )
                    ],
                  ),
                ),
              ),
            ),
          ),
        );
      },
    );
  }
}

MyApp