I’ve had this and managed to fix it, but it’s frustrating.
If it’s crashing because the default level is the problem, and you don’t care about that level, you can open your project in a file browser, and just swap the name of the problematic level with another level. They are usually in the Levels directory in your project’s files and will have a .umap file extension.
If you want to try to restore the broken level, it isn’t pretty, but I’ve worked around this by, again, opening the project in a file explorer. You would need to have autosave enabled before the crash. This wouldn’t work if you’ve disabled autosave.
There’s a World Partition directory in your project’s files, and opening this up, there’s a directory with a huge number of other directories all with two-letter names like 5Y or 4C or BJ.
One of these is causing the problem. I don’t know any way to find out which one, but if you start by moving half of these directories to a temporary place elsewhere on your computer, and try starting the engine, you can narrow it down.
If you moved half of them and it still crashes, move that half back and try temporarily moving the other half.
Once you know which half is the problem, take half of THOSE files and move them, basically a process of elimination.
This isn’t perfect, because you will lose some of your level depending what was in the removed directory, and it takes forever, but it will get you up and running again with enough patience.
This is probably caused by a null reference, so something like, if your World Partition data relies on being able to find a specific thing, like a material (maybe?) and it can’t, it handles this very badly and crashes. It seems likely to be caused by doing a force delete, or an engine crash in the middle of a regular (but perhaps large) delete, but I can’t rule out other causes.
Easiest way to avoid this problem is not to enable World Partition, but that’s quite extreme.
Another way would be to avoid doing anything that requires a force delete, until you’re happy with your project. Then back up the entire project, do all your force deletes at once, and you can just restore the entire project if you trigger this error.
Hopefully someone with more knowledge can add more or better ways to this thread, until the engine is updated to better handle these situations.