Today I was upgrading a 6 month old BabylonJS project to the latest preview and found a bug on Holographic Button - specifically on the imageUrl part. I checked with the docs if the methods has changed, but it seems like it is broken.
Official Docs for Holographic Button - Babylon 3D GUI | Babylon.js Documentation
Open the above link and open the playground attached in the docs. It will give a “Cannot read property apply of undefined” error. Switch the BabylonJS branch from latest to 4.2.0 and it should work just fine.
Another way is to just comment the line number 18 in the playground and it works in the latest release.
Don’t know if this is a bug, but definitely seems like something is broken.