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

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

 /*
  *  Copyright 2015 The WebRTC Project Authors. All rights reserved.
  *
  *  Use of this source code is governed by a BSD-style license
  *  that can be found in the LICENSE file in the root of the source
  *  tree. An additional intellectual property rights grant can be found
  *  in the file PATENTS.  All contributing project authors may
  *  be found in the AUTHORS file in the root of the source tree.
  */
 
@@ skipping 290 matching lines @@
   }
 
   @Override
   protected void onDestroy() {
     disconnect();
     if (logToast != null) {
       logToast.cancel();
     }
     activityRunning = false;
     rootEglBase.release();
-    cpuMonitor.release();
+    if (cpuMonitor != null) {
+      cpuMonitor.release();
+    }
     super.onDestroy();
   }
 
   // CallFragment.OnCallEvents interface implementation.
   @Override
   public void onCallHangUp() {
     disconnect();
   }
 
   @Override
@@ skipping 378 matching lines @@
         }
       }
     });
   }
 
   @Override
   public void onPeerConnectionError(final String description) {
     reportError(description);
   }
 }