The shape of the app bar's Material as well as its shadow.
If this property is null, then AppBarTheme.shape of ThemeData.appBarTheme is used. Both properties default to null. If both properties are null then the shape of the app bar's Material is just a simple rectangle.
A shadow is only displayed if the elevation is greater than zero.
This property is used to configure an AppBar.
final ShapeBorder? shape;