Today, I wanted to try NSMBe DS, but I was stopped by a crash at the level selection screen when it says: Loading Editor! I downloaded the 3rd and latest release of the DS editor, and I use my USA ROM (correctly renamed nsmb.nds). Maybe it is my flashcart or TWiLight Menu ++? I will try on my New 3DS XL.

I thought nsmbe for the nds was outdated? And anyways, level editing on pc would be much easier though.
If it doesn't work on your physical device, try an emulator and run it on debug mode to determine the issue.
I recommend MelonDS, and by running via emulator, you'll have an easier time determining the actual problem.
Good luck!

NSMBe on the DS? Where? I can't hack on my PC, so maybe on the DS?