Trouble calling a plugin's controller function from within an event listener function
You really, really shouldn't do this.

I'm not familiar with FormBuilder, but if it is well-built, its controllers will comprise thin methods calling services to do the grunt work; services which you can also call.

If there isn't a service method which does what you want (still assuming everything is well-architected), that means the author chose not to make that functionality part of FormBuilder's API, presumably for a good reason.

If FormBuilder isn't well-built, and instead shoves a load of code in its controllers, well, that's still not a reason for you to do something similarly ill-considered.