flexibleSpace property Null safety
This widget is stacked behind the toolbar and the tab bar. Its height will be the same as the app bar's overall height.
A flexible space isn't actually flexible unless the AppBar's container changes the AppBar's size. A SliverAppBar in a CustomScrollView changes the AppBar's height when scrolled.
Typically a FlexibleSpaceBar. See FlexibleSpaceBar for details.
final Widget? flexibleSpace;