onSelectChanged property Null safety
Called when the user selects or unselects a selectable row.
If this is not null, then the row is selectable. The current selection state of the row is given by selected.
If any row is selectable, then the table's heading row will have a checkbox that can be checked to select all selectable rows (and which is checked if all the rows are selected), and each subsequent row will have a checkbox to toggle just that row.
A row whose onSelectChanged callback is null is ignored for the purposes of determining the state of the "all" checkbox, and its checkbox is disabled.
final ValueChanged<bool?>? onSelectChanged;