Owing to the fact that my website uses https for the connection, I can not use http://maps.google.com/maps/api/js?sensor=false. When I debug the whole page, this link says: Warning : The page index.html ran insecure content. so I made another search on google and came across to this question. so what basically causes a problem is not using https link in the source part so the correct link will be (for me)
https://maps.google.com/maps/api/js?sensor=false
now everything works just fine!