I downloaded Bank's 5.1 GApps, and the latest 12.1 Nightly.
Disable any Xposed modules you may have enabled just to be safe, reboot after.
Boot into TWRP Recovery.
Backup everything — System, Data, Boot. (Do it, don't be lazy!)
Dirty flash 12.1 Nightly.
Flash GApps.
Reboot system.
Not a...