The spacing between actions when the button bar overflows.
If the widgets in actions do not fit into a single row, they are arranged into a column. This parameter provides additional vertical space in between buttons when it does overflow.
Note that the button spacing may appear to be more than the value provided. This is because most buttons adhere to the MaterialTapTargetSize of 48px. So, even though a button might visually be 36px in height, it might still take up to 48px vertically.
If null then no spacing will be added in between buttons in an overflow state.
final double actionsOverflowButtonSpacing