[BUG]: Null check operator used on a null value

[amadaluzia]

How to reproduce

1. Go to any chatroom
2. Double tap a message to select the text on it
3. Tap 'Select all'

Error log

#0      SelectableRegionState.endGlyphHeight (package:flutter/src/widgets/selectable_region.dart:1538)
#1      SelectableRegionState.contextMenuAnchors (package:flutter/src/widgets/selectable_region.dart:1378)
#2      new AdaptiveTextSelectionToolbar.selectableRegion (package:flutter/src/material/adaptive_text_selection_toolbar.dart:179)
#3      SelectionArea._defaultContextMenuBuilder (package:flutter/src/material/selection_area.dart:96)
#4      SelectableRegionState._onAnyDragEnd.<anonymous closure> (package:flutter/src/widgets/selectable_region.dart:938)
#5      SelectionOverlay.showToolbar.<anonymous closure> (package:flutter/src/widgets/text_selection.dart:1491)
#6      ContextMenuController.show.<anonymous closure> (package:flutter/src/widgets/context_menu_controller.dart:65)
#7      _OverlayEntryWidgetState.build (package:flutter/src/widgets/overlay.dart:397)
#8      StatefulElement.build (package:flutter/src/widgets/framework.dart:5729)
#9      ComponentElement.performRebuild (package:flutter/src/widgets/framework.dart:5617)
#10     StatefulElement.performRebuild (package:flutter/src/widgets/framework.dart:5780)
#11     Element.rebuild (package:flutter/src/widgets/framework.dart:5333)
#12     BuildScope._tryRebuild (package:flutter/src/widgets/framework.dart:2693)
#13     BuildScope._flushDirtyElements (package:flutter/src/widgets/framework.dart:2752)
#14     BuildOwner.buildScope (package:flutter/src/widgets/framework.dart:3048)
#15     WidgetsBinding.drawFrame (package:flutter/src/widgets/binding.dart:1162)
#16     RendererBinding._handlePersistentFrameCallback (package:flutter/src/rendering/binding.dart:468)
#17     SchedulerBinding._invokeFrameCallback (package:flutter/src/scheduler/binding.dart:1397)
#18     SchedulerBinding.handleDrawFrame (package:flutter/src/scheduler/binding.dart:1318)
#19     SchedulerBinding._handleDrawFrame (package:flutter/src/scheduler/binding.dart:1176)
#20     _invoke (dart:ui/hooks.dart:312)
#21     PlatformDispatcher._drawFrame (dart:ui/platform_dispatcher.dart:419)
#22     _drawFrame (dart:ui/hooks.dart:283)