title: Flutter | ExpansionTile property "children" throws a Bottom Overflowed issue on expanding
body: <p>I am building a list of items fetched from Firestore wrapped in an <code>ExpansionTile</code> inside a <code>ListView.builder()</code>. The data is fetched properly. No issues there. I can see the list of those items.</p>
  
  <p>The problem is on Tile expansion I need to display some <code>ListTile</code>. But on expansion, it throws me a <code>Bottom Overflowed by X pixels</code>. </p>
  
  <p>To tackle this, I tried wrapping it in a <code>ListView</code> but it throws exception -</p>
  
  <blockquote>
    <p>Another exception was thrown: NoSuchMethodError: The
    method '&lt;=' was called on null.</p>
  </blockquote>
  
  <p>No luck.</p>
  
  <p>I am coding something like this - </p>
  
  <pre><code>import 'package:cloud_firestore/cloud_firestore.dart';
  import 'package:flutter/material.dart';
  
  class NewExpansionPanel extends StatefulWidget {
    @override
    _NewExpansionPanelState createState() =&gt; _NewExpansionPanelState();
  }
  
  class _NewExpansionPanelState extends State&lt;NewExpansionPanel&gt; {
    Widget getListOfTexts(){
      return ListView(
        children: &lt;Text&gt;[
          Text('one'),
          Text('two'),
          Text('three'),
        ],
      );
    }
  
    @override
    Widget build(BuildContext context) {
      return StreamBuilder(
        stream: Firestore.instance.collection(*someCollection*).snapshots(),
        builder: (context, snapshot) {
          if(!snapshot.hasData) return const Text("Loading.....");
  
          return ListView.builder(
            itemExtent: 80.0,
            itemCount: snapshot.data.documents.length,
            itemBuilder: (context, index) {
              final DocumentSnapshot document = snapshot.data.documents[index];
              return new ExpansionTile(
                children: [
                  getListOfTexts()
                ],
                title: Column(
                  crossAxisAlignment: CrossAxisAlignment.start,
                  mainAxisAlignment: MainAxisAlignment.center,
                  children: &lt;Widget&gt;[
                    Text(
                      document['something'] ?? ' ',
                    ),
                    Text(document['title'] ?? '&lt;No title retrieved&gt;'),
                  ],
                ), 
                onExpansionChanged: (bool isExpanded) {
                  // put some code here in future
                },
              );
            }
          );
        }
      );
    }
  }
  </code></pre>