title: callback for "ext.flutter.inspector.getRootWidgetSummaryTree"
body: <p>Today when I start the flutter 2.0.x app, shows error:</p>
  <pre><code>======== Exception caught by Flutter framework =====================================================
  The following _CastError was thrown during a service extension callback for &quot;ext.flutter.inspector.getRootWidgetSummaryTree&quot;:
  Null check operator used on a null value
  
  When the exception was thrown, this was the stack: 
  #0      WidgetInspectorService._registerObjectGroupServiceExtension.&lt;anonymous closure&gt; (package:flutter/src/widgets/widget_inspector.dart:793:84)
  #1      WidgetInspectorService._registerObjectGroupServiceExtension.&lt;anonymous closure&gt; (package:flutter/src/widgets/widget_inspector.dart:792:17)
  #2      BindingBase.registerServiceExtension.&lt;anonymous closure&gt; (package:flutter/src/foundation/binding.dart:597:32)
  &lt;asynchronous suspension&gt;
  ====================================================================================================
  </code></pre>
  <p>seems flutter framework error. why did this happen? any clue about this? is it possible to fix it or avoid this eror?</p>