Deleting from a large set of text (with block quote) crashes

[mark8044]

If you have an enormous amount of text in block quote and try to highlight and then delete some (or all of it you get an error) and red screen of death

The following assertion was thrown building _OverlayEntryWidget-[LabeledGlobalKey<_OverlayEntryWidgetState>#49698](dirty, state: _OverlayEntryWidgetState#927d9):
'package:flutter_quill/src/widgets/editor.dart': Failed assertion: line 922 pos 12: 'extentChild != null': is not true.

The relevant error-causing widget was: 
  MaterialApp MaterialApp:file:///main.dart:765:14
When the exception was thrown, this was the stack: 
#2      RenderEditor.getEndpointsForSelection (package:flutter_quill/src/widgets/editor.dart:922:12)
#3      EditorTextSelectionOverlay._buildToolbar (package:flutter_quill/src/widgets/text_selection.dart:311:36)
#4      _OverlayEntryWidgetState.build (package:flutter/src/widgets/overlay.dart:208:34)
#5      StatefulElement.build (package:flutter/src/widgets/framework.dart:4870:27)
#6      ComponentElement.performRebuild (package:flutter/src/widgets/framework.dart:4754:15)
#7      StatefulElement.performRebuild (package:flutter/src/widgets/framework.dart:4928:11)
#8      Element.rebuild (package:flutter/src/widgets/framework.dart:4477:5)
#9      BuildOwner.buildScope (package:flutter/src/widgets/framework.dart:2659:19)
#10     WidgetsBinding.drawFrame (package:flutter/src/widgets/binding.dart:882:21)
#11     SchedulerBinding._invokeFrameCallback (package:flutter/src/scheduler/binding.dart:1144:15)
#12     SchedulerBinding.handleDrawFrame (package:flutter/src/scheduler/binding.dart:1081:9)
(elided 5 frames from class _AssertionError and dart:async)

[Wizzel1]

Same here

[mark8044]

It looks like #769 fixes this issue. Thank you @mtallenca !