P: Win7 Crashes instantly when extension uses script UI

Photoshop CC2015 on Win7 64bit crashes instantly when an extension runs a jsx to draw a window or palette using script UI. The same script works perfect when being run from ExtendScript instead of from the extension.

If I first run any script that draws a script UI window (either from ExtendScript or from File->Scripts) then the extension suddenly works without any crashes... until next time PS is restarted.

I have tried this on two different pc's with two different extensions, one being the default project generated by Extension Builder 3.