Fix scheduler NPE in DownloadService

PiperOrigin-RevId: 230260266

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

@@ -65,7 +65,7 @@ public class DemoDownloadService extends DownloadService {
 
   @Override
   protected void onDownloadStateChanged(DownloadState downloadState) {
-    Notification notification = null;
+    Notification notification;
     if (downloadState.state == DownloadState.STATE_COMPLETED) {
       notification =
           DownloadNotificationUtil.buildDownloadCompletedNotification(

library/core/src/main/java/com/google/android/exoplayer2/offline/DownloadService.java

@@ -462,7 +462,7 @@ public abstract class DownloadService extends Service {
     public void detachService(DownloadService downloadService, boolean unschedule) {
       Assertions.checkState(this.downloadService == downloadService);
       this.downloadService = null;
-      if (unschedule) {
+      if (scheduler != null && unschedule) {
         scheduler.cancel();
       }
     }