My approach

import 'dart:convert';
import 'package:date_time_picker/date_time_picker.dart';
import 'package:flutter/material.dart';
import 'AllPathLabTestModel.dart';
import 'package:http/http.dart' as http;

class MultipleTestBooking extends StatefulWidget {
  const MultipleTestBooking({Key? key}) : super(key: key);

  @override
  _MultipleTestBookingState createState() => _MultipleTestBookingState();
}

class _MultipleTestBookingState extends State<MultipleTestBooking> {


  void initState(){
  super.initState();
  AllPathLab();
}



  String _selectedDate = DateTime.now().toString();
  final List<String> allLabList = [
    "Select Lab",
    "Abc",
    "Xyz",
  ];
  String selectedLabFromList = "Abc";

  @override
  Widget build(BuildContext context) {
    var screenWidth = MediaQuery.of(context).size.width;
    var screenHeight = MediaQuery.of(context).size.height;
    var blockSizeHorizontal = (screenWidth / 100);
    var blockSizeVertical = (screenHeight / 100);

    return Scaffold(
      body: SafeArea(
        child: Container(
          child: Column(
            children: [
              Padding(
                padding: const EdgeInsets.all(8.0),
                child: ListTile(
                  title: Text("Booking Information",
                      style: TextStyle(
                        fontWeight: FontWeight.bold,
                        fontSize: blockSizeHorizontal * 5,
                        fontFamily: 'Poppins',
                        color: Theme.of(context).primaryColor,
                      )),
                  subtitle: Text("Preferred Visit Date"),
                ),
              ),
              //==============================================================================
              Container(
                margin: EdgeInsets.only(left: 20),
                padding: EdgeInsets.only(left: 0, right: 150),
                decoration: BoxDecoration(
                  // color: Color(0xFF3E64FF),
                  color: Colors.lightBlue[50],
                  borderRadius: BorderRadius.all(Radius.circular(12)),
                  // boxShadow: [
                  //   BoxShadow(
                  //     //color: Color(0xFF3E64FF).withOpacity(0.3),
                  //     color: Colors.grey.withOpacity(0.9),
                  //     offset: const Offset(
                  //       0.0,
                  //       5.0,
                  //     ),
                  //     blurRadius: 3.0,
                  //     spreadRadius: 0.5,
                  //   ),
                  // ],
                ),
                child: Padding(
                  padding: const EdgeInsets.all(8.0),
                  child: DateTimePicker(
                    initialValue: DateTime.now().toString(),
                    //initialValue:'', // initialValue or controller.text can be null, empty or a DateTime string otherwise it will throw an error.
                    type: DateTimePickerType.date,
                    dateLabelText: 'Select Date',
                    style: TextStyle(
                      fontWeight: FontWeight.bold,
                      fontSize: blockSizeHorizontal * 3.5,
                      fontFamily: 'Poppins',
                      color: Colors.green,
                      letterSpacing: 2.0,
                    ),
                    firstDate: DateTime.now(),
                    lastDate: DateTime.now().add(Duration(
                        days: 30)), // This will add one year from current date
                    validator: (value) {
                      return null;
                    },
                    onChanged: (value) {
                      if (value.isNotEmpty) {
                        setState(() {
                          _selectedDate = value;
                        });
                      }
                    },
                    onSaved: (value) {
                      if (value.isNotEmpty) {
                        _selectedDate = value;
                      }
                    },
                  ),
                ),
              ),

              //==============================================================================


               Container(
                      child: FutureBuilder<List<Partner>>(
                        future: AllPathLab(),
                        builder:
                            (BuildContext context, AsyncSnapshot snapshot) {
                          if (snapshot.connectionState !=ConnectionState.done) {
                            return CircularProgressIndicator();
                          }
                          if (snapshot.hasError) {
                            return Text("Somthing went wrong");
                          }

                          if (snapshot.hasData) {
                            return DropdownButton<Partner>(
                              dropdownColor: Colors.white,
                      underline: SizedBox(),
                      isExpanded: true,
                      items: snapshot.data.map((Partner data) =>
                       DropdownMenuItem<Partner>(
                         child: Text("${data.partnerName}"),
                       value: data,
                       )
                        ).toList(),
                        onChanged: (value){

                        });
                            
                          }
                          return Text("Waiting for Internet Connection");
                        },
                      ),
                    ),









              // ListTile(
              //   title: Text(
              //     " Select Pathological Lab",
              //     style: TextStyle(
              //       fontWeight: FontWeight.bold,
              //       fontSize: blockSizeHorizontal * 4.0,
              //       fontFamily: 'Poppins',
              //       color: Theme.of(context).primaryColor,
              //     ),
              //   ),
              // ),

              // Padding(
              //         padding: const EdgeInsets.only(left: 0.0),
              //         child: DropdownButton<String>(
              //           style: TextStyle(
              //             color: Colors.green,
              //             fontSize: 15,
              //             fontFamily: 'Poppins',
              //           ),
              //           value: selectedLabFromList,
              //           onChanged: (value) {
              //             setState(() {
              //               selectedLabFromList = value!;
              //             });
              //           },
              //           items:allLabList.map<DropdownMenuItem<String>>((value) {
              //             return DropdownMenuItem(
              //               child: Text(value),
              //               value: value,
              //             );
              //           }).toList(),
              //         ),
              //       ),
            ],
          ),
        ),
      ),
    );
  }
  //==========================================================================================================================================================================



  Future<List<Partner>> AllPathLab() async {
    var jsonResponse;
  
      var response = await http.post(Uri.parse("http://medbo.digitalicon.in/api/medboapi/AllPathLab"),
          body: ({
            
          }));
      if (response.statusCode == 200) {
        jsonDecode(response.body);
        jsonResponse = json.decode(response.body.toString());
        print(jsonResponse);

        AllPathLabTestModel dataModel = allPathLabTestModelFromJson(response.body);
        print(dataModel.partner.length);
        for (final item in dataModel.partner) 
        print(item.partnerName);

        List<Partner> arrData = dataModel.partner; // this "partner" is actual json array of data[]
        return arrData;
      } else {
        print("Wrong URL");
        throw Exception("Faild to fetch");
      }
    
  }





}