Allow to override `onClosing` for `ModalBottomSheet`

[vasilich6107]

Hi @jamesblasco Thanks for your work.

There is a case when I want to get true of false on closing the popup. If user clicks the button inside popup it is easy to achieve but when the popup is being closed by drag gesture there is no way to add custom return param to pop method

[jake-landersweb]

For anyone else who ever comes across this, just add a dispose() method on the view inside the widget.