java.lang.NullPointerException: Can't pass null for argument 'pathString' in child(); first time to firebase really confused : I dont really know whats wrong here, i am able to add data on the other activity.    
getting error "java.lang.NullPointerException: Can't pass null for argument 'pathString' in child()" and dont know how to fix. there are 3 other classes created only one other is adding data to firebase realtime database storage

    package com.example.akshay.katiroll.FirstScreen;
    
    import android.content.Intent;
    import android.support.v7.app.AppCompatActivity;
    import android.os.Bundle;
    import android.text.TextUtils;
    import android.view.View;
    import android.widget.Button;
    import android.widget.EditText;
    import android.widget.Toast;
    import com.example.akshay.katiroll.R;
    import com.example.akshay.katiroll.SecondScreen.welcome;
    import com.google.firebase.database.DatabaseReference;
    import com.google.firebase.database.FirebaseDatabase;
    
    public class Profile extends AppCompatActivity {
    
        EditText fName, lName, PhNo, BDay, Zip;
        Button svBtn;
        DatabaseReference mDataReference;
        String keyUser;
        String fNameStr, lNameStr, PhNoStr, BDayStr, ZipStr;
    
        @Override
        protected void onCreate(Bundle savedInstanceState) {
            super.onCreate(savedInstanceState);
            setContentView(R.layout.activity_profile);
            keyUser = getIntent().getStringExtra("USER_KEY");
            mDataReference = FirebaseDatabase.getInstance().getReference().child("Users").child(keyUser);//pretty sure something is wrong with this line
            fName = (EditText) findViewById(R.id.fName); //error in this line
            lName = (EditText) findViewById(R.id.lName);
            PhNo = (EditText) findViewById(R.id.phno);
            BDay = (EditText) findViewById(R.id.bday);
            Zip = (EditText) findViewById(R.id.zip);
            svBtn = (Button) findViewById(R.id.svBtn);
            svBtn.setOnClickListener(new View.OnClickListener() {
                @Override
                public void onClick(View v) {
    
                    fNameStr = fName.getText().toString();
                    lNameStr = lName.getText().toString();
                    PhNoStr = PhNo.getText().toString();
                    BDayStr = BDay.getText().toString();
                    ZipStr = Zip.getText().toString();
    
                    if(!TextUtils.isEmpty(fNameStr) && !TextUtils.isEmpty(lNameStr) && !TextUtils.isEmpty(PhNoStr)) {
                        DatabaseReference mDataRef = mDataReference.child("Users").push();
                        mDataRef.child("firstName").setValue(""+fNameStr);
                        mDataRef.child("lastName").setValue(""+lNameStr);
                        mDataRef.child("phoneNumber").setValue(""+PhNoStr);
                        mDataRef.child("isVerified").setValue("verified");
                        if(!TextUtils.isEmpty(BDayStr)){
                            mDataRef.child("birthday").setValue(""+BDayStr);
                        }else{ 
                            mDataRef.child("birthday").setValue("null");
                        }
                        if(!TextUtils.isEmpty(ZipStr)){
                            mDataRef.child("zipcode").setValue("ZipStr");
                        }else{
                            mDataRef.child("zipcode").setValue("null");
                        }
                        Toast.makeText(Profile.this, "User profile added", Toast.LENGTH_LONG).show();
    
                        startActivity(new Intent(Profile.this, welcome.class));
    
                    }else{
                        Toast.makeText(Profile.this, "Failed to create User Account", Toast.LENGTH_LONG).show();
    
                    }
                }
            });
        }
    }

getting error "java.lang.NullPointerException: Can't pass null for argument 'pathString' in child()" and dont know how to fix. there are 3 other classes created only one other is adding data to firebase realtime database storage

