Programmatically change Meta Description on Content Page from database using C# in ASP.Net

I have Multiple Master With Diffrent Folders .

when We Use Root Directrioes Default Page And Set Dynamic Description Then Its Working Fine .

string des = Server.HtmlDecode(ds.Tables[0].Rows[0]["course_description"].ToString());
HtmlMeta metaDescription = (HtmlMeta) Page.Master.FindControl("metaDescription");
metaDescription.Content = des;

But When We Use this Code With Another Folder Content page then Its  Show Null Refrence Exception