Remove all handler messages when controller is released

Issue: androidx/media#74
PiperOrigin-RevId: 448205908

RELEASENOTES.md

@@ -89,6 +89,8 @@
         ([#59](https://github.com/androidx/media/pull/59))
     *   Update session position info on timeline
         change([#51](https://github.com/androidx/media/issues/51))
+    *   Fix NPE in MediaControllerImplBase after releasing controller
+        ([#74](https://github.com/androidx/media/issues/74))
 
 *   Data sources:
 

libraries/session/src/main/java/androidx/media3/session/MediaControllerImplBase.java

@@ -309,6 +309,7 @@ import org.checkerframework.checker.nullness.qual.NonNull;
       serviceConnection = null;
     }
     connectedToken = null;
+    flushCommandQueueHandler.removeCallbacksAndMessages(/* token= */ null);
     this.iSession = null;
     controllerStub.destroy();
     if (iSession != null) {
@@ -3046,9 +3047,9 @@ import org.checkerframework.checker.nullness.qual.NonNull;
     }
 
     public void sendFlushCommandQueueMessage() {
-      // Send message to notify the end of the transaction. It will be handled when the current
-      // looper iteration is over.
-      if (!hasMessages(MSG_FLUSH_COMMAND_QUEUE)) {
+      if (iSession != null && !hasMessages(MSG_FLUSH_COMMAND_QUEUE)) {
+        // Send message to notify the end of the transaction. It will be handled when the current
+        // looper iteration is over.
         sendEmptyMessage(MSG_FLUSH_COMMAND_QUEUE);
       }
     }