title: ".MainActivity" in <activity android:name=".MainActivity"> is not a concrete class
body: <p>When inspecting my code, I get an error in the AndroindManifest.xml saying that </p>
  
  <blockquote>
    <p>".MainActivity" in &lt;activity android:name=".MainActivity"&gt; is not a concrete class.</p>
  </blockquote>
  
  <p>The App is, therefore, crashing in the Emulator of Android Studio. </p>
  
  <p>Here is the Java Code of the Application:</p>
  
  <pre><code>package de.thi.donotpressthebutton;
  
  import android.media.MediaPlayer; 
  import android.support.v7.app.AppCompatActivity; 
  import android.os.Bundle;
  import android.view.View;
  import android.widget.Button;
  
  public class MainActivity extends AppCompatActivity {
      Button button;
  
      @Override
      protected void onCreate(Bundle savedInstanceState) {
          super.onCreate(savedInstanceState);
          setContentView(R.layout.activity_main);
  
          button = (Button) findViewById(R.id.button);
          final MediaPlayer audio = MediaPlayer.create(this, R.raw.niggaz_audio );
          button.setOnClickListener(new View.OnClickListener() {
              @Override
              public void onClick(View view) {
                  audio.start();
              }
          });
      }
  
  }
  </code></pre>
  
  <p>And here is the XML Code: </p>
  
  <pre><code>&lt;?xml version="1.0" encoding="utf-8"?&gt;
  &lt;manifest xmlns:android="http://schemas.android.com/apk/res/android"
      package="de.thi.donotpressthebutton"&gt;
  
      &lt;application
          android:allowBackup="true"
          android:icon="@mipmap/ic_launcher"
          android:label="@string/app_name"
          android:roundIcon="@mipmap/ic_launcher_round"
          android:supportsRtl="true"
          android:theme="@style/AppTheme"&gt;
          &lt;activity android:name=".MainActivity"&gt; **//error**
              &lt;intent-filter&gt;
                  &lt;action android:name="android.intent.action.MAIN" /&gt;
  
                  &lt;category android:name="android.intent.category.LAUNCHER" /&gt;
              &lt;/intent-filter&gt;
          &lt;/activity&gt;
      &lt;/application&gt;
  
  &lt;/manifest&gt;
  </code></pre>