Im on the same OS, 10.12.2.
So here is the exact sequence. The file that contains the form is currently secret, form a different file I setactivedatabase to “Estimates” (which contains the form I want to print) and opensavedwindows.
This opens my main Estimates form. From this form, I open the form I want to mail a pdf from using the View menu, I then select Print One Record from the File menu and I select to Mail a PDF.
Hmmmm, now it is not giving me an error. I have quit and relaunched Panorama X but it is still not throwing the error. I will have to look closer what I am doing when the error shows up. I do have 2 monitors, my main laptop and a connected second monitor which I have noticed might cause some weird things to happen.
I will get back to you.