I was having black screen issues too after transferring my code from phone to emulator this morning
I immediately backed up the FGO folders in Android/data and Android/obb (LDPlayer)
Then I reinstalled the modded app
It took a number of tries but after getting into the game and downloading data I knew the app was working, maybe the blackscreen was caused by the server not recognizing my transfer
So then I closed the app, deleted the newly created FGO folders in Android/data and Android/obb
Then I made backups of the backup folders I originally copied (just in case)
Then I renamed the original backup to what the newly created folder name was (i.e. com.fategrandorder etc.). That's the expected name of the folder
Then I restarted the game. It took a couple of tries but I was able to get to the loading screen and I saw my code in the bottom right
When I tapped on start, I got a message but I just tapped the right side button anyway. And I was able to get into my account & immediately created a new backup code