You never initialize mAuth
anywhere, so when you then call createUserWithEmailAndPassword
on it it indeed is invalid.
You'll want to initialize mAuth
in onCreate
like:
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_register);
mAuth = FirebaseAuth.getInstance();
EditText id = findViewById(R.id.id);
EditText pass = findViewById(R.id.pass);
Also see: