Cannot access a property or method of a null object reference
Means that your code references an object in it which is null. It could be it hasn’t been created, or it could be it has been deleted. Common reasons include things like failing to load something your code relies on, or not being able to create something because of some resource limit being reached.
I can’t help you with Adobe Animate I’m afraid, I have not used it for many years and even when I did I mostly avoided the timeline. Your best bet if you can’t identify the problem code is run it with the debug player so it stops at the point of the error, and you can examine it to see what is missing/null.