Hello. If you play Pokémon Ultra-Sun with the Citra emulator, you can have a black screen by entering the castle of the Festival Plaza. I found a solution to solve the problem:
-Install Citra for Android on your phone (https://github.com/SachinVin/citra_android)
-Install Pokemon on your phone (the same version as on your computer)
-Launch Citra Android, indicate the location of Pokemon and play until you can save the game (to create the storage path)
-Quit Citra and connect the phone to your computer.
-Replace the Pokemon backup of your phone (citra-emu/title/…/main) with the backup of your computer.
-Play Pokemon on your phone and visit the Festival Plaza (there should be no black screen), leave the Festival Plaza and save.
-Now, replace the backup on your computer by the backup on your phone.
You can play Pokemon on your PC without any problem.