Flutter: Checkbox Tristate won't work on Null Safety

how can i archive tristate for checkbox with nullsafety.... i have to pass null into that value but nullsafety has to perform null check on a variable, this is just contradicting .....

putting null check as usual:

                 bool? parentvalue;

                 void update() {
                     parentvalue = null;
                  }


                 Checkbox(
                      ....
                      value: parentvalue!,
                  onChanged: update(),
                      ....
                    ),



ERROR: Null check operator used on a null value

if i remove null check...code cannot compile at all

                  bool parentvalue;

                  void update() {
                    parentvalue = null;
                    }


                  Checkbox(
                      ....
                      value: parentvalue,
                      onChanged: update(),
                      ....
                    ),

ERROR: A value of type 'Null' can't be assigned to a variable of type 'bool'.