the log.e is causing a null pointer exception

I'm on a different project but am using this for it's login info

I'm trying to send a message to the log with the logged in username if I'm logged in, and send the user to the login screen if not. When I try to run my code I get a nullpointer exception that looks as such:

01-07 00:08:25.314 12048-12048/com.parse.starter E/AndroidRuntime: 

FATAL EXCEPTION: main

Process: com.parse.starter, PID: 12048

java.lang.RuntimeException: Unable to start activity ComponentInfo{com.parse.starter/com.parse.starter.MainActivity}: java.lang.NullPointerException: println needs a message at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2356) at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2418) at android.app.ActivityThread.access$900(ActivityThread.java:154) at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1321) at android.os.Handler.dispatchMessage(Handler.java:102) at android.os.Looper.loop(Looper.java:135) at android.app.ActivityThread.main(ActivityThread.java:5289) at java.lang.reflect.Method.invoke(Native Method) at java.lang.reflect.Method.invoke(Method.java:372) at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:904) at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:699) Caused by: java.lang.NullPointerException: println needs a message at android.util.Log.println_native(Native Method) at android.util.Log.i(Log.java:160) at com.parse.starter.MainActivity.onCreate(MainActivity.java:43) at android.app.Activity.performCreate(Activity.java:5990) at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1106) at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2309) at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2418) at android.app.ActivityThread.access$900(ActivityThread.java:154) at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1321) at android.os.Handler.dispatchMessage(Handler.java:102) at android.os.Looper.loop(Looper.java:135) at android.app.ActivityThread.main(ActivityThread.java:5289) at java.lang.reflect.Method.invoke(Native Method) at java.lang.reflect.Method.invoke(Method.java:372)

I tried adding a '+ ""' to the end of getUserName, and that allowed the app to run, but I couldn't find the only username I've made anywhere in the log, so I can't tell if I've done this correctly and am logged in. I also tried adding .toString() after getUserName but the app still didn't run. Thank you in advace :-)

    package com.parse.starter;

    import android.app.Activity;
    import android.content.Intent;
    import android.os.Bundle;
    import android.support.v7.app.ActionBarActivity;
    import android.support.v7.app.AppCompatActivity;
    import android.util.Log;
    import android.view.Menu;
    import android.view.MenuItem;

    import com.parse.ParseAnalytics;
    import com.parse.ParseObject;
    import com.parse.ParseUser;



    public class MainActivity extends AppCompatActivity {
    public static final String TAG = MainActivity.class.getSimpleName();


    protected void onCreate(Bundle savedInstanceState) {

    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_main);
    ParseAnalytics.trackAppOpenedInBackground(getIntent());

    ParseUser currentUser = ParseUser.getCurrentUser();
      if (currentUser == null) {
      Intent intent = new Intent(this, LoginActivity.class);
      intent.addFlags(intent.FLAG_ACTIVITY_NEW_TASK);
      intent.addFlags(intent.FLAG_ACTIVITY_CLEAR_TASK);
      startActivity(intent);
      } else {
      Log.i(TAG, currentUser.getUsername());

      }
      }

    public boolean onCreateOptionsMenu (Menu menu){
    // Inflate the menu; this adds items to the action bar if it is present.
    getMenuInflater().inflate(R.menu.menu_main, menu);
     return true;
    }

      public boolean onOptionsItemSelected (MenuItem item) {
      // Handle action bar item clicks here. The action bar will
      // automatically handle clicks on the Home/Up button, so long
      // as you specify a parent activity in AndroidManifest.xml.
      int id = item.getItemId();

      //noinspection SimplifiableIfStatement
     if (id == R.id.action_settings) {
       return true;
     }

      return super.onOptionsItemSelected(item);
    }
      }