Potential memory leak from Animation Controllers

[david-shortman]

════════ Exception caught by widgets library ═══════════════════════════════════════════════════════
The following assertion was thrown while finalizing the widget tree:
_ProgressButtonState#99899(tickers: tracking 2 tickers) was disposed with an active Ticker.

...

Tickers used by AnimationControllers should be disposed by calling dispose() on the AnimationController itself. Otherwise, the ticker will leak.

When disposing of a widget that contains a button actively being animated, the above error appears.

This widget should dispose of its AnimationControllers itself.