Fix (invisible) crash when episode does not have chapters (#8628)

### Description

Fix (invisible) crash when episode does not have chapters. Add proper
null check.

### Checklist
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app/src/main/java/de/danoeh/antennapod/ui/screen/playback/audio/CoverFragment.java

@@ -62,6 +62,8 @@ import org.greenrobot.eventbus.EventBus;
 import org.greenrobot.eventbus.Subscribe;
 import org.greenrobot.eventbus.ThreadMode;
 
+import java.util.List;
+
 import static android.widget.LinearLayout.LayoutParams.MATCH_PARENT;
 import static android.widget.LinearLayout.LayoutParams.WRAP_CONTENT;
 
@@ -222,9 +224,10 @@ public class CoverFragment extends Fragment {
     }
 
     private void refreshChapterData(int chapterIndex) {
-        if (chapterIndex > -1) {
-            if (media.getPosition() > media.getDuration() || chapterIndex >= media.getChapters().size() - 1) {
-                displayedChapterIndex = media.getChapters().size() - 1;
+        List<Chapter> chapters = media.getChapters();
+        if (chapterIndex > -1 && chapters != null) {
+            if (media.getPosition() > media.getDuration() || chapterIndex >= chapters.size() - 1) {
+                displayedChapterIndex = chapters.size() - 1;
                 viewBinding.butNextChapter.setVisibility(View.INVISIBLE);
             } else {
                 displayedChapterIndex = chapterIndex;