animation property Null safety
An animation whose value represents the current position of the TabBar's selected tab indicator as well as the scrollOffsets of the TabBar and TabBarView.
The animation's value ranges from 0.0 to length - 1.0. After the selected tab is changed, the animation's value equals index. The animation's value can be offset by +/- 1.0 to reflect TabBarView drag scrolling.
If this TabController was disposed, then return null.
Animation<double>? get animation => _animationController?.view;