Hi there!
Even the demo project has this error in Godot 3.3+
It feels like this project is abandoned a while ago but I'll still try :D
How to reproduce this error:
It's quite easy: open demo project with Godot 3.3, build it to your device and check logs with adb for this error
05-28 14:03:22.683 32653 626 E godot : **SCRIPT ERROR**: Invalid call. Nonexistent function 'init' in base 'JNISingleton'.