When you add or remove a property tag from a property tag category, the following error is displayed:
You have entered some invalid input. This is usually because a mandatory field is missing, or a unique field has been duplicated. The message below provides more information.
may not be null
This was also caused by changes in BZ#1116705, so setting it as a duplicate.
The cause was that in the old version the sort property wasn't being set because the PropertyTagToPropertyTagCategory was being constructed behind the scenes. To fix this I manually reconstructed it, but forgot to set the propertyTagCategory field to the entity.
*** This bug has been marked as a duplicate of bug 1116705 ***