I am trying to make a forget password activity where there is an EditText field for user to input their e-mail and click a Button and the sendPasswordResetEmail() method will be invoked... So the code that I have is this

mAuth = FirebaseAuth.getInstance();
resetPasswordBtn = (Button)findViewById(R.id.resetPasswordButton);
resetPasswordFld = (EditText)findViewById(R.id.resetPasswordField);

final String email = resetPasswordFld.getText().toString().trim();

resetPasswordBtn.setOnClickListener(new View.OnClickListener() {
    @Override
    public void onClick(View view) {
        mAuth.sendPasswordResetEmail(email)
                .addOnCompleteListener(new OnCompleteListener() {
                    @Override
                    public void onComplete(@NonNull Task task) {
                        if (task.isSuccessful()) {
                            Toast.makeText(ResetPassword.this, "e-mail buat reset password sudah dikirimkan",Toast.LENGTH_LONG).show();
                            Intent loginIntent = new Intent (ResetPassword.this, LoginActivity.class);
                            startActivity(loginIntent);
                        }
                    }
                });
    }
});

When I tried it, the app crashes, and I got this error log

03-23 16:28:59.829 6990-6990E/UncaughtException: java.lang.IllegalArgumentException: Given String is empty or null
    at com.google.android.gms.common.internal.zzac.zzdv(Unknown Source)
    at com.google.firebase.auth.FirebaseAuth.sendPasswordResetEmail(Unknown Source)
    at com.ResetPassword$1.onClick(ResetPassword.java:37)
    at android.view.View.performClick(View.java:6207)
    at android.widget.TextView.performClick(TextView.java:11094)
    at android.view.View$PerformClick.run(View.java:23639)
    at android.os.Handler.handleCallback(Handler.java:751)
    at android.os.Handler.dispatchMessage(Handler.java:95)
    at android.os.Looper.loop(Looper.java:154)
    at android.app.ActivityThread.main(ActivityThread.java:6688)
    at java.lang.reflect.Method.invoke(Native Method)
    at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:1468)
    at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:1358)
03-23 16:28:59.833 6990-7024 D/FA: Logging event (FE): _ae, Bundle[{_o=crash, _sc=ResetPassword, _si=-8440426569227389093, timestamp=1490261339829, fatal=1}]
03-23 16:29:00.128 6990-6990 E/AndroidRuntime: FATAL EXCEPTION: main
 Process: , PID: 6990
 java.lang.IllegalArgumentException: Given String is empty or null
 at com.google.android.gms.common.internal.zzac.zzdv(Unknown Source)
 at com.google.firebase.auth.FirebaseAuth.sendPasswordResetEmail(Unknown Source)
 at.ResetPassword$1.onClick(ResetPassword.java:37)
 at android.view.View.performClick(View.java:6207)
 at android.widget.TextView.performClick(TextView.java:11094)
 at android.view.View$PerformClick.run(View.java:23639)
 at android.os.Handler.handleCallback(Handler.java:751)
 at android.os.Handler.dispatchMessage(Handler.java:95)
 at android.os.Looper.loop(Looper.java:154)
 at android.app.ActivityThread.main(ActivityThread.java:6688)
 at java.lang.reflect.Method.invoke(Native Method)
 at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:1468)
 at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:1358)

I am actually convinced that I am doing this wrong as mAuth will definitely returns null as no user is logged in right? I am having difficulties finding good documentation about this and the documentation in the firebase documentations does not really help.

There must be a way to reference a database to check if the email address inputted really exists... But my rules prevent users that aren't logged in to access them... So I am just really confused on how to get started with this...