isCompleted property Null safety
Whether the future has been completed.
Reflects whether complete or completeError has been called.
true value doesn't necessarily mean that listeners of this future
have been invoked yet, either because the completer usually waits until
a later microtask to propagate the result, or because complete
was called with a future that hasn't completed yet.
When this value is
true, complete and completeError must not be
bool get isCompleted;