Add dispose for animation controller for scrollable_positioned_list

[kishansr1]

Issue

While startScroll is getting called we are creating AnimationController but we are not disposing that controller.

Solution

Dispose that controller

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[jamesderlin]

It's not the main part of this PR, but:

@xster There are going to keep being PRs and issues about nullable vs. non-nullable SchedulerBinder.instance thing until #339 (comment) is addressed.

[kishansr1]

@jamesderlin I think now it can be merged.

[xster]

Sorry for the long wait. I've created something similar at #423 to address the same issue. The fix should be on 0.3.4 on pub now.

[BenDevExatech]

Hi,
I came up here searching for a solution for an error : "_ScrollablePositionedListState#44259(tickers: tracking 2 tickers) was disposed with an active Ticker".

I'm on 0.3.4 version of scrollable_positioned_list package, and I still get this error.

Is this fix generic and autonomous, or should I call a dispose somewhere, as ItemScrollController class still doesn't have any dispose function?

My context: scrollable_positioned_list is used on a pop-up. If the pop-up is closed while a scrollTo event is still in progress, I get the error.

Thanks in advance for your help, tips or any solution.