Update to reuse existing window, if it exists

Author: keveleigh

Assets/MRTK/Core/Inspectors/Setup/MRTKVersionPopup.cs

@@ -20,7 +20,8 @@ private static void Init()
         {
             if (window != null)
             {
-                window.Close();
+                window.ShowUtility();
+                return;
             }
 
             window = CreateInstance<MRTKVersionPopup>();