title: MultiDexApplication class in secondary dex file not found when using AndroidX
body: <p>I'm using <code>'androidx.multidex:multidex:2.0.0'</code> and I'm pointing to the MultiDexApplication class from the manifest using:</p>
  
  <pre><code>&lt;application
      android:name="androidx.multidex.MultiDexApplication"
      ...
  </code></pre>
  
  <p>On the build.gradle I also added the follwoing entry:</p>
  
  <pre><code>defaultConfig {
      ...
      multiDexEnabled true
      ...
  </code></pre>
  
  <p>When running the application in Android versions older than 5.0 (API 21) I get the following exception:</p>
  
  <pre><code>java.lang.RuntimeException: Unable to instantiate application androidx.multidex.MultiDexApplication: java.lang.ClassNotFoundException: Didn't find class "androidx.multidex.MultiDexApplication" on path [...]
  </code></pre>
  
  <p>I used the APK browser and I saw that the MultiDexApplication class and the other MultiDex helper classes where in the classes2.dex file. That's probably why it does'nt find the class.</p>
  
  <p>Must I force those classes to be on the main classes.dex file? How can this be fixed?</p>