Adding item to sequence is realized by dispatching `add()` to the sequence actor. However, this could race with the sequence actor destruction.:
After the dispatch but before the dispatched `add()` message gets processed by the sequence actor, if the sequence gets destroyed, a terminate message will be injected to the head of the message queue. This would result in the destruction of the sequence without the `add()` call ever gets processed. User would end up with a pending future and the future's `onDiscarded' would not be triggered during the sequence destruction.
The solution is to set the `inject` flag to `false` so that the terminating message is enqueued to the end of the sequence actor message queue. All `add()` messages that happen before the destruction will be processed before the terminating message.