Going full screen causes chewieController to be disposed()

```
I/flutter ( 8811): ══╡ EXCEPTION CAUGHT BY GESTURE ╞═══════════════════════════════════════════════════════════════════
I/flutter ( 8811): The following assertion was thrown while handling a gesture:
I/flutter ( 8811): A ChewieController was used after being disposed.
I/flutter ( 8811): Once you have called dispose() on a ChewieController, it can no longer be used.
I/flutter ( 8811):
I/flutter ( 8811): When the exception was thrown, this was the stack:
I/flutter ( 8811): #0 ChangeNotifier._debugAssertNotDisposed.
package:flutter/…/foundation/change_notifier.dart:105
I/flutter ( 8811): #1 ChangeNotifier._debugAssertNotDisposed
package:flutter/…/foundation/change_notifier.dart:111
I/flutter ( 8811): #2 ChangeNotifier.notifyListeners
package:flutter/…/foundation/change_notifier.dart:200
I/flutter ( 8811): #3 ChewieController.toggleFullScreen
package:chewie/src/chewie_player.dart:325
I/flutter ( 8811): #4 _MaterialControlsState._onExpandCollapse.
package:chewie/src/material_controls.dart:300
I/flutter ( 8811): #5 State.setState
package:flutter/…/widgets/framework.dart:1122
I/flutter ( 8811): #6 _MaterialControlsState._onExpandCollapse
package:chewie/src/material_controls.dart:297
I/flutter ( 8811): #7 GestureRecognizer.invokeCallback
package:flutter/…/gestures/recognizer.dart:120
I/flutter ( 8811): #8 TapGestureRecognizer._checkUp
package:flutter/…/gestures/tap.dart:242
I/flutter ( 8811): #9 TapGestureRecognizer.acceptGesture
package:flutter/…/gestures/tap.dart:204
I/flutter ( 8811): #10 GestureArenaManager.sweep
package:flutter/…/gestures/arena.dart:156
I/flutter ( 8811): #11 _WidgetsFlutterBinding&BindingBase&GestureBinding.handleEvent
package:flutter/…/gestures/binding.dart:218
I/flutter ( 8811): #12 _WidgetsFlutterBinding&BindingBase&GestureBinding.dispatchEvent
package:flutter/…/gestures/binding.dart:192
I/flutter ( 8811): #13 _WidgetsFlutterBinding&BindingBase&GestureBinding._handlePointerEvent
package:flutter/…/gestures/binding.dart:149
I/flutter ( 8811): #14 _WidgetsFlutterBinding&BindingBase&GestureBinding._flushPointerEventQueue
package:flutter/…/gestures/binding.dart:101
I/flutter ( 8811): #15 _WidgetsFlutterBinding&BindingBase&GestureBinding._handlePointerDataPacket
package:flutter/…/gestures/binding.dart:85
I/flutter ( 8811): #19 _invoke1 (dart:ui/hooks.dart:223:10)
I/flutter ( 8811): #20 _dispatchPointerDataPacket (dart:ui/hooks.dart:144:5)
I/flutter ( 8811): (elided 3 frames from package dart:async)
I/flutter ( 8811):
I/flutter ( 8811): Handler: onTap
I/flutter ( 8811): Recognizer:
I/flutter ( 8811): TapGestureRecognizer#05ede(debugOwner: GestureDetector, state: ready, won arena, finalPosition:
I/flutter ( 8811): Offset(800.3, 388.9), sent tap down)
I/flutter ( 8811): ════════════════════════════════════════════════════════════════════════════════════════════════════
```