"The getter 'position' was called on null" throws when using a FadingEdgeScrollView inside the TabBarView

[shemhazai]

A null pointer exception is thrown when using the FadingEdgeScrollView inside the TabBarView.

Steps to reproduce:

1. Clone this repo with a minimum code which reproduces the problem: shemhazai/flutter_fading_edge_scrollview
2. Navigate through pages by clicking on the TabBar tabs (must not be adjacent tabs).
3. The exception is thrown (as posted below).

I/flutter ( 6149): ══╡ EXCEPTION CAUGHT BY SCHEDULER LIBRARY >╞═════════════════════════════════════════════════════════
I/flutter ( 6149): The following NoSuchMethodError was thrown during a scheduler callback:
I/flutter ( 6149): The getter 'position' was called on null.
I/flutter ( 6149): Receiver: null
I/flutter ( 6149): Tried calling: position
I/flutter ( 6149):
I/flutter ( 6149): When the exception was thrown, this was the stack:
I/flutter ( 6149): #0 Object.noSuchMethod (dart:core-patch/object_patch.dart:53:5)
I/flutter ( 6149): #1 _FadingEdgeScrollViewState.initState. (package:fading_edge_scrollview/src/fading_edge_scrollview.dart:168:23)
I/flutter ( 6149): #2 SchedulerBinding._invokeFrameCallback (package:flutter/src/scheduler/binding.dart:1113:15)
I/flutter ( 6149): #3 SchedulerBinding.handleDrawFrame (package:flutter/src/scheduler/binding.dart:1060:9)
I/flutter ( 6149): #4 SchedulerBinding._handleDrawFrame (package:flutter/src/scheduler/binding.dart:968:5)
I/flutter ( 6149): #8 _invoke (dart:ui/hooks.dart:261:10)
I/flutter ( 6149): #9 _drawFrame (dart:ui/hooks.dart:219:3)
I/flutter ( 6149): (elided 3 frames from dart:async)

I did not experience this exception when I was using the FadingEdgeScrollView without the TabBarView - so most likely this is an important factor to consider.

[mponkin]

Turns out in this situation PostFrameCallback happens after the widget is disposed. So I added check for this.
Fix is available in version 1.1.4
Thank you for pointing out this problem and providing such helpful description