Fix NPE cause by demo app adding null listener to drm manager

Also added an assertion to the DRM event dispatcher to cause
immediate failure when this happens. This is consistent with
the assertion in the equivalent MediaSource class.

-------------
Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=191892735

demos/main/src/main/java/com/google/android/exoplayer2/demo/PlayerActivity.java

@@ -42,7 +42,6 @@ import com.google.android.exoplayer2.PlaybackPreparer;
 import com.google.android.exoplayer2.Player;
 import com.google.android.exoplayer2.SimpleExoPlayer;
 import com.google.android.exoplayer2.drm.DefaultDrmSessionManager;
-import com.google.android.exoplayer2.drm.DrmSessionManager;
 import com.google.android.exoplayer2.drm.FrameworkMediaCrypto;
 import com.google.android.exoplayer2.drm.FrameworkMediaDrm;
 import com.google.android.exoplayer2.drm.HttpMediaDrmCallback;
@@ -120,7 +119,6 @@ public class PlayerActivity extends Activity
   }
 
   private Handler mainHandler;
-  private EventLogger eventLogger;
   private PlayerView playerView;
   private LinearLayout debugRootView;
   private TextView debugTextView;
@@ -296,7 +294,7 @@ public class PlayerActivity extends Activity
         return;
       }
 
-      DrmSessionManager<FrameworkMediaCrypto> drmSessionManager = null;
+      DefaultDrmSessionManager<FrameworkMediaCrypto> drmSessionManager = null;
       if (intent.hasExtra(DRM_SCHEME_EXTRA) || intent.hasExtra(DRM_SCHEME_UUID_EXTRA)) {
         String drmLicenseUrl = intent.getStringExtra(DRM_LICENSE_URL);
         String[] keyRequestPropertiesArray = intent.getStringArrayExtra(DRM_KEY_REQUEST_PROPERTIES);
@@ -341,17 +339,21 @@ public class PlayerActivity extends Activity
       trackSelector = new DefaultTrackSelector(adaptiveTrackSelectionFactory);
       trackSelectionHelper = new TrackSelectionHelper(trackSelector, adaptiveTrackSelectionFactory);
       lastSeenTrackGroupArray = null;
-      eventLogger = new EventLogger(trackSelector);
 
       player =
           ExoPlayerFactory.newSimpleInstance(renderersFactory, trackSelector, drmSessionManager);
       player.addListener(new PlayerEventListener());
+
+      EventLogger eventLogger = new EventLogger(trackSelector);
       player.addListener(eventLogger);
       player.addMetadataOutput(eventLogger);
       player.addAudioDebugListener(eventLogger);
       player.addVideoDebugListener(eventLogger);
-      player.setPlayWhenReady(shouldAutoPlay);
+      if (drmSessionManager != null) {
+        drmSessionManager.addListener(mainHandler, eventLogger);
+      }
 
+      player.setPlayWhenReady(shouldAutoPlay);
       playerView.setPlayer(player);
       playerView.setPlaybackPreparer(this);
       debugViewHelper = new DebugTextViewHelper(player, debugTextView);
@@ -372,7 +374,8 @@ public class PlayerActivity extends Activity
           releaseAdsLoader();
           loadedAdTagUri = adTagUri;
         }
-        MediaSource adsMediaSource = createAdsMediaSource(mediaSource, Uri.parse(adTagUriString));
+        MediaSource adsMediaSource =
+            createAdsMediaSource(mediaSource, Uri.parse(adTagUriString), eventLogger);
         if (adsMediaSource != null) {
           mediaSource = adsMediaSource;
         } else {
@@ -426,8 +429,8 @@ public class PlayerActivity extends Activity
     }
   }
 
-  private DrmSessionManager<FrameworkMediaCrypto> buildDrmSessionManagerV18(UUID uuid,
-      String licenseUrl, String[] keyRequestPropertiesArray, boolean multiSession)
+  private DefaultDrmSessionManager<FrameworkMediaCrypto> buildDrmSessionManagerV18(
+      UUID uuid, String licenseUrl, String[] keyRequestPropertiesArray, boolean multiSession)
       throws UnsupportedDrmException {
     HttpMediaDrmCallback drmCallback = new HttpMediaDrmCallback(licenseUrl,
         buildHttpDataSourceFactory(false));
@@ -440,7 +443,6 @@ public class PlayerActivity extends Activity
     DefaultDrmSessionManager<FrameworkMediaCrypto> drmSessionManager =
         new DefaultDrmSessionManager<>(
             uuid, FrameworkMediaDrm.newInstance(uuid), drmCallback, null, multiSession);
-    drmSessionManager.addListener(mainHandler, eventLogger);
     return drmSessionManager;
   }
 
@@ -455,7 +457,6 @@ public class PlayerActivity extends Activity
       mediaSource = null;
       trackSelector = null;
       trackSelectionHelper = null;
-      eventLogger = null;
     }
   }
 
@@ -494,7 +495,8 @@ public class PlayerActivity extends Activity
   }
 
   /** Returns an ads media source, reusing the ads loader if one exists. */
-  private @Nullable MediaSource createAdsMediaSource(MediaSource mediaSource, Uri adTagUri) {
+  private @Nullable MediaSource createAdsMediaSource(
+      MediaSource mediaSource, Uri adTagUri, EventLogger eventLogger) {
     // Load the extension source using reflection so the demo app doesn't have to depend on it.
     // The ads loader is reused for multiple playbacks, so that ad playback can resume.
     try {

library/core/src/main/java/com/google/android/exoplayer2/drm/DefaultDrmSessionEventListener.java

@@ -17,6 +17,7 @@ package com.google.android.exoplayer2.drm;
 
 import android.os.Handler;
 import com.google.android.exoplayer2.Player;
+import com.google.android.exoplayer2.util.Assertions;
 import java.util.concurrent.CopyOnWriteArrayList;
 
 /** Listener of {@link DefaultDrmSessionManager} events. */
@@ -57,6 +58,7 @@ public interface DefaultDrmSessionEventListener {
 
     /** Adds listener to event dispatcher. */
     public void addListener(Handler handler, DefaultDrmSessionEventListener eventListener) {
+      Assertions.checkArgument(handler != null && eventListener != null);
       listeners.add(new HandlerAndListener(handler, eventListener));
     }