Flutter: "a non-null string must be provided to a text widget."

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Flutter

Hello, i get the following error message:

data != null
"A non-null String must be provided to a Text widget."

I dont know why i get this error message, please help me...

<pre>import 'package:flutter/material.dart';

class UserHome extends StatefulWidget {
  final String title;

  UserHome({Key key, this.title}) : super(key: key);

  @override
  _MyHomePageState createState() => _MyHomePageState();

}



class _MyHomePageState extends State<UserHome> {
  int _selectedDestination = 0;

  @override
  Widget build(BuildContext context) {
    final theme = Theme.of(context);
    final textTheme = theme.textTheme;

    return MaterialApp(
      home: Scaffold(
      body: Row(
        children: [
          Drawer(
            child: ListView(
              // Important: Remove any padding from the ListView.
              padding: EdgeInsets.zero,
              children: <Widget>[
                Padding(
                  padding: const EdgeInsets.all(16.0),
                  child: Text(
                    'Header',
                    style: textTheme.headline6,
                  ),
                ),
                Divider(
                  height: 1,
                  thickness: 1,
                ),
                ListTile(
                  leading: Icon(Icons.favorite),
                  title: Text('Item 1'),
                  selected: _selectedDestination == 0,
                  onTap: () => selectDestination(0),
                ),
                ListTile(
                  leading: Icon(Icons.delete),
                  title: Text('Item 2'),
                  selected: _selectedDestination == 1,
                  onTap: () => selectDestination(1),
                ),
                ListTile(
                  leading: Icon(Icons.label),
                  title: Text('Item 3'),
                  selected: _selectedDestination == 2,
                  onTap: () => selectDestination(2),
                ),
                Divider(
                  height: 1,
                  thickness: 1,
                ),
                Padding(
                  padding: const EdgeInsets.all(16.0),
                  child: Text(
                    'Label',
                  ),
                ),
                ListTile(
                  leading: Icon(Icons.bookmark),
                  title: Text('Item A'),
                  selected: _selectedDestination == 3,
                  onTap: () => selectDestination(3),
                ),
              ],
            ),
          ),
          VerticalDivider(
            width: 1,
            thickness: 1,
          ),
          Expanded(
            child: Scaffold(
              appBar: AppBar(
                title: Text(widget.title),
              ),
              body: GridView.count(
                crossAxisCount: 2,
                crossAxisSpacing: 20,
                mainAxisSpacing: 20,
                padding: EdgeInsets.all(20),
                childAspectRatio: 3 / 2,
                children: [
                  Image.asset('assets/nav-drawer-1.svg'),
                  Image.asset('assets/nav-drawer-2.svg'),
                  Image.asset('assets/nav-drawer-3.svg'),
                  Image.asset('assets/nav-drawer-4.svg'),
                ],
              ),
            ),
          ),
        ],
      ),
    ),
    );
  }

  void selectDestination(int index) {
    setState(() {
      _selectedDestination = index;
    });
  }
}

What I have tried:

I searched for this topic in the internet and found out that a variable is empty but i dont know which of these i using it is.