In my local development, working on a plugin, I get a similarly strange behavior with respect to the composer.
The create topic button works in my local discourse instance, but then if I try to add a new create topic button somewhere else (and add some code to make that happen), it messes up, and all create topic buttons throughout the (local) site stop working.
And I get this error:
Uncaught Error: Assertion Failed: The key provided to set must be a string or number, you passed undefined
This continues even if I remove the code I added that seemed to cause the error in the first place. That is, once I get this error, even if I return all the code back to the way it was before, the error keeps coming up anytime I want to hit the create topic button.
The only thing that, I think, makes the error goes away, is if I remove the file entirely where the new code was (so, not having the file there–seems to work; having the file there with no code in it–the error continues).
Does anyone have any idea about this? It’s been a big hindrance to my development.