The Editor Crashes when i activate the mask editor
I encountered this a lot when developing the plugin. It happens on some old meshes that were imported before the latest engine skeletal mesh changes that occurred sometime in the last couple of engine releases. The only way I found to fix it is to re-import your mesh.
Just clicking re-import in persona sometimes works, but if that doesn't work, you will need to export the mesh to FBX and re-import it manually so the right data gets generated.
