Bugfix: MediaPlayer creation might fail for ttsFallback.

Seen on a Fairphone 4.

Part of #963.

src/main/java/de/dennisguse/opentracks/services/announcement/VoiceAnnouncement.java

@@ -126,10 +126,14 @@ public class VoiceAnnouncement {
         }
         if (ttsFallback == null) {
             ttsFallback = MediaPlayer.create(context, R.raw.tts_fallback);
+            if (ttsFallback == null) {
+                Log.w(TAG, "MediaPlayer for ttsFallback could not be created.");
+            } else {
                 ttsFallback.setAudioStreamType(AUDIO_STREAM);
                 ttsFallback.setLooping(false);
             }
         }
+    }
 
     public void announce(@NonNull Track track) {
         synchronized (this) {
@@ -148,9 +152,13 @@ public class VoiceAnnouncement {
         }
 
         if (!ttsReady) {
+            if (ttsFallback == null) {
+                Log.w(TAG, "MediaPlayer for ttsFallback was not created.");
+            } else {
                 Log.i(TAG, "TTS not ready/available, just generating a tone.");
                 ttsFallback.seekTo(0);
                 ttsFallback.start();
+            }
             return;
         }