This is different from the request for "someone else is answering" I've seen mentioned and rejected before: Indication if somebody has started to write an answer
This is something that happened to me twice, and it's fairly irritating. I start answering a question, but by the time I hit "Post your answer" the question's gone, the OP deleted it. On both occasions it was about perfectly reasonable questions. One was re-worded and re-posted 5 minutes later, the other one was deleted because someone suggested the question might be off-topic. It wasn't off-topic.
I have no idea how often this is happening, but I'd guess the "fix" might be within reach. Some sort of Ajax interaction is happening while the answer editor is being used, because we do get a "Question deleted" banner. Maybe the same mechanism can be used to let the OP know the question he's about to delete is being actively answered. Obviously the OP should still have the power to delete the question, regardless of the "actively being answered" flag, but give them the power to decide.