Fixed: crash when opening a platformWindow with a popup blocker

Previously, when opening a CPPlatformWindow a crash happened when this new window was blocked by the browser (addblock, or browser feature).
Now when opening an external window, we check if the DOMWindow has been created.

This PR has modified the method isVisible of CPPlatformWindow. We now check if the _DOMWindow is not NULL and undefined.

AppKit/Platform/CPPlatformWindow.j

@@ -213,7 +213,7 @@ var PrimaryPlatformWindow   = NULL;
 - (BOOL)isVisible
 {
 #if PLATFORM(DOM)
-    return _DOMWindow !== NULL;
+    return _DOMWindow !== NULL && _DOMWindow !== undefined;
 #else
     return NO;
 #endif

AppKit/Platform/DOM/CPPlatformWindow+DOM.j

@@ -568,6 +568,9 @@ _CPPlatformWindowWillCloseNotification = @"_CPPlatformWindowWillCloseNotificatio
 
     _DOMWindow = window.open("about:blank", "_blank", "menubar=no,location=no,resizable=yes,scrollbars=no,status=no,left=" + CGRectGetMinX(_contentRect) + ",top=" + CGRectGetMinY(_contentRect) + ",width=" + CGRectGetWidth(_contentRect) + ",height=" + CGRectGetHeight(_contentRect));
 
+    if (!_DOMWindow)
+        return;
+
     [PlatformWindows addObject:self];
 
     // FIXME: cpSetFrame?
@@ -1414,6 +1417,9 @@ _CPPlatformWindowWillCloseNotification = @"_CPPlatformWindowWillCloseNotificatio
 
 - (void)order:(CPWindowOrderingMode)orderingMode window:(CPWindow)aWindow relativeTo:(CPWindow)otherWindow
 {
+    if (!_DOMWindow)
+        return;
+
     [CPPlatform initializeScreenIfNecessary];
 
     // Grab the appropriate level for the layer, and create it if