FATAL EXCEPTION: main?

FATAL EXCEPTION: main
                                                                      Process: com.x.x, PID: 15829
                                                                      java.lang.NoClassDefFoundError: com.x.x.activities.NewItemActivity
                                                                          at com.x.x.activities.MainActivity.startNewItemActivity(MainActivity.java:31)
                                                                          at com.x.x.activities.MainActivity.access$000(MainActivity.java:10)
                                                                          at com.x.x.activities.MainActivity$1.onClick(MainActivity.java:24)
                                                                          at android.view.View.performClick(View.java:4764)
                                                                          at android.view.View$PerformClick.run(View.java:19844)
                                                                          at android.os.Handler.handleCallback(Handler.java:739)
                                                                          at android.os.Handler.dispatchMessage(Handler.java:95)
                                                                          at android.os.Looper.loop(Looper.java:135)
                                                                          at android.app.ActivityThread.main(ActivityThread.java:5351)
                                                                          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:908)
                                                                          at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:703)

public class MainActivity extends ListActivity {

    private ImageView mAddItemImage;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);

        mAddItemImage = (ImageView) findViewById(R.id.AddItemImage);

        mAddItemImage.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
                startNewItemActivity();
            }
        });

    }

    private void startNewItemActivity() {
        Intent intent = new Intent(this, NewItemActivity.class);
        startActivity(intent);
    }
}

public class NewItemActivity extends AppCompatActivity {

    private ImageView mNewNoteImage;
    private ImageView mNewCategoryImage;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_new_item);

        mNewNoteImage = (ImageView) findViewById(R.id.NewNoteImage);
        mNewCategoryImage = (ImageView) findViewById(R.id.NewCategoryImage);
    }
}