Bug 556524. Check for nsWindow existence before creating taskbar preview. r=roc a=blocker

browser/components/wintaskbar/WindowsPreviewPerTab.jsm

@@ -459,7 +459,13 @@ TabWindow.prototype = {
                   .QueryInterface(Ci.nsIInterfaceRequestor)
                   .getInterface(Ci.nsIWebNavigation)
                   .QueryInterface(Ci.nsIDocShell);
-    let preview = AeroPeek.taskbar.createTaskbarTabPreview(docShell, controller);
+    let preview;
+    try {
+      preview = AeroPeek.taskbar.createTaskbarTabPreview(docShell, controller);
+    } catch (e) {
+      controller.destroy();
+      return;
+    }
     preview.visible = AeroPeek.enabled;
     preview.active = this.tabbrowser.selectedTab == tab;
     // Grab the default favicon

widget/src/windows/WinTaskbar.cpp

@@ -359,6 +359,11 @@ WinTaskbar::CreateTaskbarTabPreview(nsIDocShell *shell, nsITaskbarPreviewControl
   if (!toplevelHWND)
     return NS_ERROR_INVALID_ARG;
 
+  nsWindow *window = nsWindow::GetNSWindowPtr(toplevelHWND);
+
+  if (!window)
+    return NS_ERROR_INVALID_ARG;
+
   nsRefPtr<TaskbarTabPreview> preview(new TaskbarTabPreview(mTaskbar, controller, toplevelHWND, shell));
   if (!preview)
     return NS_ERROR_OUT_OF_MEMORY;