isPropagationStopped property Null safety
Indicates if the propagation of this event has been stopped.
If true, processing of any event listeners in nodes subsequent to the
current node in the event flow is prevented. This does not affect any
event listeners in the current node (currentTarget). In contrast,
stopsImmediatePropagation indicates if processing of event listeners
in both the current node and subsequent nodes is prevented.
bool get isPropagationStopped => _isPropagationStopped;