*This is only relevant if you have custom materials/textures in your project.*
This may be quite obvious for a lot of people, but I ran into this issue and Googling yielded not much help, and I discovered this by accident while rebuilding the project again from scratch. So at the cost of looking like an idiot, here's a quick tip.
If your MSFS permanently freezes (but not crash) as soon as you click "Load in Editor" for your BGL scenery asset group and all the sub-menus pop up, make sure you have loaded your MaterialLib FIRST. I had custom materials and did not click the "Load in Editor" for my MaterialLib first, hence when I loaded the BGL scenery, the sim froze. By loading in MaterialLib first, I mean wait until it is all loaded and check in the materials editor. The sim has a chance of freezing otherwise.
If there are solutions around this, I will update the post with the most current and elegant solution.