Proper using of question mark (null safety) in flutter

Both versions should end up doing the same thing. When null-safety enabled, ?. short-circuits the rest of the expression, so if the receiver is null, the rest of the expression (including the right-hand-side of the assignment) won't be evaluated. You can observe that in the following example:

class Foo {
  int value = 0;
}

int bar() {
  print('Called bar');
  return 42;
}

void main() {
  Foo? foo = null;
  foo?.value = bar();
}

where running it generates no errors and shows that bar() is never called.

That said, the second version using ?. is much less clear (which is evident from the apparent confusion in the several deleted answers) and therefore probably should be avoided.

The best way would be to introduce a local variable:

final mapPickerController = widget.mapPickerController;
if (mapPickerController != null){
  mapPickerController.mapMoving = mapMoving;
  mapPickerController.mapFinishedMoving = mapFinishedMoving;
}

and then you don't need to use the null-assertion operator at all, and it's slightly more efficient since you check for null only once.