I think the error message I pasted that happened when opening a .j3md file could be unrelated to the issue with the visual material editor not opening properly.
I do not get any error message when opening the material editor, it just looks blank like this when I open any material that uses any of JME’s stock shaders such as pbr or unshaded.
And I can still edit the material’s text file if I click the “source” tab without any issues, its just the display for the visual editor that’s missing.
My project that’s having this issue was also created with the sdk’s basic gradle game template quite a while ago using the sdk 3.2 release.