Do not require subtitleButton in custom layouts of StyledPlayerView

Every other subtitleButton has an if not null check, but does not force non null.

Issue: #7962
PiperOrigin-RevId: 334124323

RELEASENOTES.md

@@ -11,6 +11,9 @@
         ([#7866](https://github.com/google/ExoPlayer/issues/7866)).
 *   Text:
     *   Add support for `\h` SSA/ASS style override code (non-breaking space).
+*   UI:
+    *   Do not require subtitleButton in custom layouts of StyledPlayerView
+        ([#7962](https://github.com/google/ExoPlayer/issues/7962)).
 *   Extractors:
     *   Add support for .mp2 boxes in the `AtomParsers`
         ([#7967](https://github.com/google/ExoPlayer/issues/7967)).

library/ui/src/main/java/com/google/android/exoplayer2/ui/StyledPlayerControlView.java

@@ -2005,11 +2005,13 @@ public class StyledPlayerControlView extends FrameLayout {
           break;
         }
       }
-      checkNotNull(subtitleButton)
-          .setImageDrawable(subtitleIsOn ? subtitleOnButtonDrawable : subtitleOffButtonDrawable);
-      checkNotNull(subtitleButton)
-          .setContentDescription(
-              subtitleIsOn ? subtitleOnContentDescription : subtitleOffContentDescription);
+
+      if (subtitleButton != null) {
+        subtitleButton.setImageDrawable(
+            subtitleIsOn ? subtitleOnButtonDrawable : subtitleOffButtonDrawable);
+        subtitleButton.setContentDescription(
+            subtitleIsOn ? subtitleOnContentDescription : subtitleOffContentDescription);
+      }
       this.rendererIndices = rendererIndices;
       this.tracks = trackInfos;
       this.mappedTrackInfo = mappedTrackInfo;