If true the checkbox's value can be true, false, or null.
Checkbox displays a dash when its value is null.
When a tri-state checkbox is tapped its onChanged callback will be applied to true if the current value is null or false, false otherwise. Typically tri-state checkboxes are disabled (the onChanged callback is null) so they don't respond to taps.
If tristate is false (the default), value must not be null.
final bool tristate