I just ran into the same issue. It seems like I moved the Material Editor off the edge of the screen which seemed to try and make the editor a separate window…but it froze. Every time I opened the project the Material Editor appeared in the same place and the game froze.
I was able to fix it by restarting MSFS, creating a new project, opening the Material Editor and then resizing it. Then, when I re-opened my original project, the problem was solved.