dispose method
Discards any resources used by the object.
This method should not remove its overlayEntries from the Overlay. The object's owner is in charge of doing that.
After this is called, the object is not in a usable state and should be discarded.
This method should only be called by the object's owner; typically the Navigator owns a route and so will call this method when the route is removed, after which the route is no longer referenced by the navigator.
Implementation
@override
void dispose() {
_sheetController.dispose();
super.dispose();
}