Don't run postexecute if activity is gone.

Change-Id: I9133a4d9e4e0fd2a92cc4df786e0bc5407d981fe
Fixes: 36551766
Test: monkey tests

src/com/android/settings/notification/AppNotificationSettings.java

@@ -102,6 +102,9 @@ public class AppNotificationSettings extends NotificationSettingsBase {
 
             @Override
             protected void onPostExecute(Void unused) {
+                if (getHost() == null) {
+                    return;
+                }
                 populateChannelList();
             }
         }.execute();