One simple reason you may find this error is when in one of your callbacks you use a component name that doesn’t exist.
In my case, I misspelled, in a State clause, my component name: instead of “session_dir” I typed “session-dir”.
Unfortunate for me that I didn’t follow my own naming conventions.
I see in the forum several situations that I believe are special cases of this type of mistake.
It seems there are other causes, though. This is at least something simple to check for.