The Set String Value action does not accept an empty string.
In most use cases this is a good check, as it shows an error and reminds you to fill that field out. (In most cases you want something there.)
But I wanted to use it to empty a string variable. I can't have even a space character in it.
Should there be a Empty String Value action?
Luckily I am not stuck, as the Set Fsm String does the job. It allows an empty value.