null reference error when widget height is unconstrained

Nobody has claimed this yet.

Hey. I have no idea why is this happening, but when I put Marquee directly in my widget tree instead of `Text` widget it gives a null reference with a long call stack:
```
═════ Exception caught by rendering library ═════════════════════════════════
Null check operator used on a null value
The relevant error-causing widget was
CustomScrollView
lib\pages\subtitle_details.dart:182
════════════════════════════════════════════════════════════════════════════════
E/flutter (31099): [ERROR:flutter/lib/ui/ui_dart_state.cc(199)] Unhandled Exception: Null check operator used on a null value
E/flutter (31099): #0 ScrollPosition.minScrollExtent
package:flutter/…/widgets/scroll_position.dart:136
E/flutter (31099): #1 ScrollPosition._updateSemanticActions
package:flutter/…/widgets/scroll_position.dart:634
E/flutter (31099): #2 ScrollPosition.notifyListeners
package:flutter/…/widgets/scroll_position.dart:929
E/flutter (31099): #3 ScrollPosition.forcePixels
package:flutter/…/widgets/scroll_position.dart:377
E/flutter (31099): #4 ScrollPositionWithSingleContext.jumpTo
package:flutter/…/widgets/scroll_position_with_single_context.dart:198
E/flutter (31099): #5 ScrollController.jumpTo
package:flutter/…/widgets/scroll_controller.dart:173
...
E/flutter (31099): #8 SchedulerBinding._invokeFrameCallback
package:flutter/…/scheduler/binding.dart:1144
E/flutter (31099): #9 SchedulerBinding.handleDrawFrame
package:flutter/…/scheduler/binding.dart:1090
E/flutter (31099): #10 SchedulerBinding._handleDrawFrame
package:flutter/…/scheduler/binding.dart:998
E/flutter (31099): #11 _rootRun (dart:async/zone.dart:1354:13)
E/flutter (31099): #12 _CustomZone.run (dart:async/zone.dart:1258:19)
E/flutter (31099): #13 _CustomZone.runGuarded (dart:async/zone.dart:1162:7)

═════ Exception caught by scheduler library ═════════════════════════════════
Null check operator used on a null value
════════════════════════════════════════════════════════════════════════════════
E/flutter (31099): #14 _invoke (dart:ui/hooks.dart:163:10)
E/flutter (31099): #15 PlatformDispatcher._drawFrame (dart:ui/platform_dispatcher.dart:259:5)
E/flutter (31099): #16 _drawFrame (dart:ui/hooks.dart:126:31)
E/flutter (31099):
E/flutter (31099): [ERROR:flutter/lib/ui/ui_dart_state.cc(199)] Unhandled Exception: Null check operator used on a null value
E/flutter (31099): #0 RenderViewportBase.hitTestChildren
package:flutter/…/rendering/viewport.dart:703
E/flutter (31099): #1 RenderBox.hitTest
package:flutter/…/rendering/box.dart:2414
E/flutter (31099): #2 RenderProxyBoxMixin.hitTestChildren
package:flutter/…/rendering/proxy_box.dart:131
....
E/flutter (31099): #84 GestureBinding._handlePointerDataPacket
package:flutter/…/gestures/binding.dart:279
E/flutter (31099): #85 _rootRunUnary (dart:async/zone.dart:1370:13)
E/flutter (31099): #86 _CustomZone.runUnary (dart:async/zone.dart:1265:19)
E/flutter (31099): #87 _CustomZone.runUnaryGuarded (dart:async/zone.dart:1170:7)
E/flutter (31099): #88 _invoke1 (dart:ui/hooks.dart:182:10)
E/flutter (31099): #89 PlatformDispatcher._dispatchPointerDataPacket (dart:ui/platform_dispatcher.dart:282:7)
E/flutter (31099): #90 _dispatchPointerDataPacket (dart:ui/hooks.dart:96:31)
```
But when I put my widget in a SizedBox with a fixed height, it works, but I need to know exact height of font to make it work in this way. scrolling axis is horizontal so it should not care about vertical bounds, so I guess it's a bug in marquee.

Reproduce the failure by placing Marquee directly in an unconstrained widget tree, using the fixed-height SizedBox comparison and the setup at lib/pages/subtitle_details.dart:182 as reference. Inspect the reported CustomScrollView and ScrollPosition stack; done means horizontal scrolling works without a null-reference exception when no fixed height is supplied.

Written by the indexing model from the issue text.