Hi mohammed
Indeed reverting to None after you have saved and run on a different value can still lead to the same errors. What worked for me to solve it if it re-occured is add a new Open Page action in there that opened the same page but do not change the close page settings (so keep at none).
simply deleting and recreating the showpage action and not touching the 'closePage' settings will work
try this and comment if the error is solved or not.it is a bug in mendix I think if your issues solved let me know..
regards
Bon Van Heijster
Gurumoorthy