Alright so the lowerbody NIF was a bodypart from the mod Room207 A4 Male Body Replacer, the modeling app that I used is Blender 2.93.2., and I imported the NIF file after installing NIFstools for blender. I started off by imported the model directly from the mod itself as a NIF, sculpt the model into the shape I wanted, and then exported back into a separate folder in which I sent to the _male folder to replace the original NIF. After that I just booted up the game and when I unequip the pants to see if everything was working properly, then game crashes.