Got the same problem as you have.
Could not click Theme-It in Form Builder 2 nore on the Theme-It button it self after creating a simpel form.
Unistalled the Theme It extention then tryed Theme It in Formbuilder 2 on that form i did, and it worked.
Tryed Theme-It on another page not created by FormBuilder 2 and it worked fine without any errors.
Will do some more tests.