I'm running on an iPhone 8 with iOS 13.3 on the latest Checkra1n 0.9.7 beta.
I'm using FLEXall as my FLEX loader (I tried FLEXer, with the same results, leading me to believe that this is an issue with PermaFlex)
Here is a video demonstration. In it, I disabled all Cydia substrate addons except FLEXall and PermaFlex, yet it still crashes when I click "Add new property/ivar."
Thanks for the help :) Your tweak is fantastic.