Don't try to access runtime after determining it is null. Pull request by mduggan.

desktopsharing.js

@@ -95,6 +95,7 @@ function checkExtInstalled(isInstalledCallback) {
     if (!chrome.runtime) {
         // No API, so no extension for sure
         isInstalledCallback(false);
+        return;
     }
     chrome.runtime.sendMessage(
         config.chromeExtensionId,