null_argument_to_non_null_type

'{0}' shouldn't be called with a 'null' argument for the non-nullable type argument '{1}'.

The analyzer produces this diagnostic when null is passed to either the constructor Future.value or the method Completer.complete when the type argument used to create the instance was non-nullable. Even though the type system can't express this restriction, passing in a null results in a runtime exception.

The following code produces this diagnostic because null is being passed to the constructor Future.value even though the type argument is the non-nullable type String:

Future<String> f() {
  return Future.value(null);
}

Pass in a non-null value:

Future<String> f() {
  return Future.value('');
}