onSelectAll property Null safety
Invoked when the user selects or unselects every row, using the checkbox in the heading row.
If this is null, then the DataRow.onSelectChanged callback of every row in the table is invoked appropriately instead.
To control whether a particular row is selectable or not, see DataRow.onSelectChanged. This callback is only relevant if any row is selectable.
final ValueSetter<bool?>? onSelectAll;