--- a/browser/modules/ContentWebRTC.jsm
+++ b/browser/modules/ContentWebRTC.jsm
@@ -131,17 +131,18 @@ function handleGUMStop(aSubject, aTopic,
 
   let request = {
     windowID: aSubject.windowID,
     rawID: aSubject.rawID,
     mediaSource: aSubject.mediaSource,
   };
 
   let mm = getMessageManagerForWindow(contentWindow);
-  mm.sendAsyncMessage("webrtc:StopRecording", request);
+  if (mm)
+    mm.sendAsyncMessage("webrtc:StopRecording", request);
 }
 
 function handleGUMRequest(aSubject, aTopic, aData) {
   let constraints = aSubject.getConstraints();
   let secure = aSubject.isSecure;
   let contentWindow = Services.wm.getOuterWindowWithId(aSubject.windowID);
 
   contentWindow.navigator.mozGetUserMediaDevices(