Flutter App dies with "Failed assertion: line 4901 pos 16: 'child is! ParentDataElement<ParentData>': is not true."

Checking the logs you've provided, the error seems to point on MediaControlWidget.dart.

════════ (5) Exception caught by widgets library ═══════════════════════════════════════════════════'package:flutter/src/widgets/framework.dart': Failed assertion: line 4901 pos 16: 'child is! ParentDataElement<ParentData>': is not true.The relevant error-causing widget was:   Expanded file:///C:/Users/narmo/Documents/theBigApp/TheBigApp/remote_appv1/lib/MediaControlWidget.dart:28:9════════════════════════════════════════════════════════════════════════════════════════════════════════════ (6) Exception caught by widgets library ═══════════════════════════════════════════════════'package:flutter/src/widgets/framework.dart': Failed assertion: line 4901 pos 16: 'child is! ParentDataElement<ParentData>': is not true.The relevant error-causing widget was:   Expanded file:///C:/Users/narmo/Documents/theBigApp/TheBigApp/remote_appv1/lib/MediaControlWidget.dart:70:9════════════════════════════════════════════════════════════════════════════════════════════════════

I've tried running error-causing the snippet, and the ones causing the issue is how Spacer is used.

From your code, Spacer is used inside an Expanded widget, similar to this code.

Column(  mainAxisAlignment: MainAxisAlignment.center,  children: <Widget>[    Expanded(flex: 1, child: Spacer()),    Expanded(flex: 1, child: Container(color: Colors.greenAccent)),    Expanded(flex: 1, child: Spacer()),  ],),

Since Spacer is an "Expanded widget", it should be used alongside Expanded widgets. It should be used similarly to this code.

Column(  mainAxisAlignment: MainAxisAlignment.center,  children: <Widget>[    Spacer(flex: 1),    Expanded(flex: 1, child: Container(color: Colors.greenAccent)),    Spacer(flex: 1),  ],),