Fix logic for CreateXamlWindow.

Author: MouriNaruto

NanaBox/Utils.cpp

@@ -309,6 +309,10 @@ HWND CreateXamlWindow(
         nullptr,
         nullptr,
         nullptr);
+    if (!WindowHandle)
+    {
+        return nullptr;
+    }
     if (!::SetWindowSubclass(
         WindowHandle,
         [](