I had a bit of a play, this might help. If a modulation already exists then double clicking clears it without the crash (also modulations can be added back to just those controls). A rough guess the crash is due to the app trying to remove an element from an array that is not there or maybe the array has not been created correctly?
Oddly there appeared fairly random behaviours, I could remove the two modulations from stream 1 time and pitch and then add modulations only to stream 2 balance and stream 3 balance. It seemed to be set to only allowing 2 modulations at a time. I found a similar one where random 2 will allow any modulation destination but only 4 at a time.
I'll check out the beta, thanks Daniel