Fixes a bug where AppRTC Android Demo crashes with empty roomId.

BUG=
R=magjed@webrtc.org

Review URL: https://codereview.webrtc.org/1975743002 .

Cr-Commit-Position: refs/heads/master@{#12759}

webrtc/examples/androidapp/src/org/appspot/apprtc/CallActivity.java

@@ -308,7 +308,9 @@ public class CallActivity extends Activity
     }
     activityRunning = false;
     rootEglBase.release();
+    if (cpuMonitor != null) {
       cpuMonitor.release();
+    }
     super.onDestroy();
   }