title: Flutter NestedScrollView error: Failed assertion: line 4944 pos 14: '_dependents.isEmpty': is not true
body: <p>I'm trying to use a NestedScrollView but am running into issues. I've reduced the code down to this simple example, showcasing the issue.</p>
  
  <pre><code>var tabs = ['Tab1', 'Tab2', 'Tab3'];
  @override
    Widget build(BuildContext context) {
      return DefaultTabController(
        length: 3,
        child: NestedScrollView(
          headerSliverBuilder: (BuildContext context, bool innerBoxIsScrolled) {
            return [
              SliverAppBar(
                pinned: true,
                expandedHeight: 200.0,
                bottom: TabBar(
                  tabs: tabs.map((tab) =&gt; Tab(text: tab)).toList(),
                ),
              )
            ];
          },
          body: TabBarView(
            children: [
              _wrapTabWidget(Text('Tab 1')),
              _wrapTabWidget(Text('Tab 2')),
              _wrapTabWidget(Text('Tab 3'))
            ],
          ),
        ),
      );
    }
  
    _wrapTabWidget(tabWidget) {
      return Builder(
        builder: (BuildContext context) {
          return CustomScrollView(
            slivers: &lt;Widget&gt;[
              SliverOverlapInjector(
                handle: NestedScrollView.sliverOverlapAbsorberHandleFor(context),
              ),
              tabWidget
            ],
          );
        },
      );
    }
  </code></pre>
  
  <p>The error I'm getting is:</p>
  
  <pre><code>'package:flutter/src/widgets/framework.dart': Failed assertion: line 4944 pos 14: '_dependents.isEmpty': is not true.
  The relevant error-causing widget was: 
    NestedScrollView
  </code></pre>
  
  <p>I've not used a NestedScrollView before, so I'm assuming that I'm missing something simple, but I'm not sure what.</p>