Bug 726272 - check if currentDocumentChannel is a valid channel; r=bustage-fix

browser/base/content/browser-thumbnails.js

@@ -110,6 +110,10 @@ let gBrowserThumbnails = {
 
     let channel = aBrowser.docShell.currentDocumentChannel;
 
+    // No valid document channel. We shouldn't take a screenshot.
+    if (!channel)
+      return false;
+
     // Don't take screenshots of internally redirecting about: pages.
     // This includes error pages.
     if (channel.originalURI.schemeIs("about"))