We have had this issue raised in the community in the past and have already put the issue with our developers. FYI,
The good news is that, the feature is already ready with our testing and it should be shortly released to both our servers viz. OCHA and HHI. Will also let you know, once when the same is released.
Kindly please be informed that the hidden question type is now supported in the form builder UI as well and thus, you should not see the error message that you have been seeing previously.