title: flutter: widget.controller != null : is not true
body: <p>when I use widget <code>Scrollbar</code> to add a scroll bar to my <code>ListView</code>, this assertion was thrown although the layout behaves normally. Complete info and related code are below:</p>
  <pre><code>══╡ EXCEPTION CAUGHT BY SCHEDULER LIBRARY ╞═
  I/flutter ( 4453): The following assertion was thrown during a scheduler callback:
  I/flutter ( 4453): 'package:flutter/src/material/scrollbar.dart': Failed assertion: line 111 pos 20: 'widget.controller
  I/flutter ( 4453): != null': is not true.
  I/flutter ( 4453):
  I/flutter ( 4453): Either the assertion indicates an error in the framework itself, or we should provide substantially
  I/flutter ( 4453): more information in this error message to help you determine and fix the underlying cause.
  I/flutter ( 4453): In either case, please report this assertion by filing a bug on GitHub:
  I/flutter ( 4453):   https://github.com/flutter/flutter/issues/new?template=BUG.md
  I/flutter ( 4453):
  I/flutter ( 4453): When the exception was thrown, this was the stack:
  I/flutter ( 4453): #2      _ScrollbarState.didChangeDependencies.&lt;anonymous closure&gt; (package:flutter/src/material/scrollbar.dart:111:20)
  I/flutter ( 4453): #3      SchedulerBinding._invokeFrameCallback (package:flutter/src/scheduler/binding.dart:1113:15)
  I/flutter ( 4453): #4      SchedulerBinding.handleDrawFrame (package:flutter/src/scheduler/binding.dart:1060:9)
  I/flutter ( 4453): #5      SchedulerBinding._handleDrawFrame (package:flutter/src/scheduler/binding.dart:968:5)
  I/flutter ( 4453): #9      _invoke (dart:ui/hooks.dart:261:10)
  I/flutter ( 4453): #10     _drawFrame (dart:ui/hooks.dart:219:3)
  I/flutter ( 4453): (elided 5 frames from class _AssertionError and dart:async)
  I/flutter ( 4453): ════════════════════════════════════════════════════════════════════════════════════════════════════
  I/flutter ( 4453): Another exception was thrown: 'package:flutter/src/material/scrollbar.dart': Failed assertion: line 111 pos 20: 'widget.controller !=
   null': is not true.
  </code></pre>
  <pre><code>Scrollbar(
                            isAlwaysShown: true,
                            child: ListView(
                              controller: sc,
                              scrollDirection: Axis.vertical,
                              padding: EdgeInsets.fromLTRB(ScreenUtil.dp(22), 0, ScreenUtil.dp(23), ScreenUtil.dp(8.13)),
                              children: &lt;Widget&gt;[
                                Html(data: &quot;&quot;&quot;$html&quot;&quot;&quot;,),
                              ],
                            ),
                          ),
                        ),
  </code></pre>