Progress indicator, used here: https://www.flutterclutter.dev/flutter/tutorials/displaying-a-prorgess-indicator/2020/37/

progress_indicator_02.dart

Container(
  padding: EdgeInsets.all(16),
  color: Colors.black.withOpacity(0.8),
  child: Column(
    mainAxisAlignment: MainAxisAlignment.center,
    mainAxisSize: MainAxisSize.min,
    children: [
      _getLoadingIndicator(),
      _getHeading(),
      _getText('Text')
    ]
  )
)

Widget _getLoadingIndicator() {
  return Padding(
    child: Container(
        child: CircularProgressIndicator(
            strokeWidth: 3
        ),
        width: 32,
        height: 32
    ),
    padding: EdgeInsets.only(bottom: 16)
  );
}

Widget _getHeading() {
  return Padding(
    child: Text(
      'Please wait …',
      style: TextStyle(
        color: Colors.white,
        fontSize: 16
      ),
      textAlign: TextAlign.center,
    ),
    padding: EdgeInsets.only(bottom: 4)
  );
}

Widget _getText(String displayedText) {
  return Text(
    displayedText,
    style: TextStyle(
        color: Colors.white,
        fontSize: 14
    ),
    textAlign: TextAlign.center,
  );
}