Got Assertion Error when scrolling the Upper layer of bottom sheet

[kartdroid]

Describe the bug
When i was scrolling the bottom Sheet , i got the below exception

To Reproduce
Steps to reproduce the behavior:

1. Random Error

Expected behavior
A clear and concise description of what you expected to happen.

Screenshots

I/flutter (22346): ══╡ EXCEPTION CAUGHT BY GESTURE ╞═══════════════════════════════════════════════════════════════════
I/flutter (22346): The following assertion was thrown while handling a gesture:
I/flutter (22346): 'package:rubber/src/bottom_sheet.dart': Failed assertion: line 158 pos 14: '_drag == null': is not
I/flutter (22346): true.
I/flutter (22346):
I/flutter (22346): Either the assertion indicates an error in the framework itself, or we should provide substantially
I/flutter (22346): more information in this error message to help you determine and fix the underlying cause.
I/flutter (22346): In either case, please report this assertion by filing a bug on GitHub:
I/flutter (22346): https://github.com/flutter/flutter/issues/new?template=BUG.md
I/flutter (22346):
I/flutter (22346): When the exception was thrown, this was the stack:
I/flutter (22346): #2 _RubberBottomSheetState._onVerticalDragDown (package:rubber/src/bottom_sheet.dart:158:14)
I/flutter (22346): #3 DragGestureRecognizer.addPointer. (package:flutter/src/gestures/monodrag.dart:123:46)
I/flutter (22346): #4 GestureRecognizer.invokeCallback (package:flutter/src/gestures/recognizer.dart:102:24)
I/flutter (22346): #5 DragGestureRecognizer.addPointer (package:flutter/src/gestures/monodrag.dart:123:9)
I/flutter (22346): #6 RawGestureDetectorState._handlePointerDown (package:flutter/src/widgets/gesture_detector.dart:698:18)
I/flutter (22346): #7 RenderPointerListener.handleEvent (package:flutter/src/rendering/proxy_box.dart:2519:14)
I/flutter (22346): #8 _WidgetsFlutterBinding&BindingBase&GestureBinding.dispatchEvent (package:flutter/src/gestures/binding.dart:158:22)
I/flutter (22346): #9 _WidgetsFlutterBinding&BindingBase&GestureBinding._handlePointerEvent (package:flutter/src/gestures/binding.dart:138:7)
I/flutter (22346): #10 _WidgetsFlutterBinding&BindingBase&GestureBinding._flushPointerEventQueue (package:flutter/src/gestures/binding.dart:101:7)
I/flutter (22346): #11 _WidgetsFlutterBinding&BindingBase&GestureBinding._handlePointerDataPacket (package:flutter/src/gestures/binding.dart:85:7)
I/flutter (22346): #12 _invoke1 (dart:ui/hooks.dart:173:13)
I/flutter (22346): #13 _dispatchPointerDataPacket (dart:ui/hooks.dart:127:5)
I/flutter (22346): (elided 2 frames from class _AssertionError)
I/flutter (22346):
I/flutter (22346): Handler: onDown
I/flutter (22346): Recognizer:
I/flutter (22346): VerticalDragGestureRecognizer#30ccf(debugOwner: GestureDetector)
I/flutter (22346): ════════════════════════════════════════════════════════════════════════════════════════════════════
I/flutter (22346): Another exception was thrown: 'package:rubber/src/bottom_sheet.dart': Failed assertion: line 219 pos 14: '_drag == null': is not true.

Smartphone (please complete the following information):

• Device: Pixel 2
• OS: Android
• Browser: NA
• Version 0.2.6

Additional context
Add any other context about the problem here.

[CristianDRM]

I'm getting this error too

[DK15]

This PR fixes above issue.

[srcho8]

i have similar problem

════════ Exception caught by gesture ═════════
══════════════════════════════════
The following _CastError was thrown while handling a gesture:
Null check operator used on a null value

When the exception was thrown, this was the stack
#0 RubberAnimationController.stop
package:rubber/src/animation_controller.dart:500
#1 RubberAnimationController.value=
package:rubber/src/animation_controller.dart:254
#2 RubberBottomSheetState._onVerticalDragUpdate
package:rubber/src/bottom_sheet.dart:261
#3 DragGestureRecognizer._checkUpdate.
package:flutter/…/gestures/monodrag.dart:436
#4 GestureRecognizer.invokeCallback
package:flutter/…/gestures/recognizer.dart:182
...
Handler: "onUpdate"
Recognizer: VerticalDragGestureRecognizer#22e78
debugOwner: GestureDetector
start behavior: start
════════════════════════════════════════════════════════════════════════════════

════════ Exception caught by gesture ═══════════════════════════════════════════
Null check operator used on a null value
════════════════════════════════════════════════════════════════════════════════