ASP.Net GridView1.Rows[rowIndex]: Object reference not set to an instance of an object.

hello all what is wrong in this scrip ..

if I use masterpage, it certainly error, do not use tap if the master page there is no error ..

Thanks,,

email: brazaganssx@yahoo.com

Description: An unhandled exception occurred during the execution of the current web request. Please review the stack trace for more information about the error and where it originated in the code.  Exception Details: System.NullReferenceException: Object reference not set to an instance of an object. Source Error: 

Line 27:             int rowIndex = int.Parse(Request.QueryString["RowIndex"]);
Line 28:             GridView GridView1 = (GridView)this.Page.PreviousPage.FindControl("GridView1");
Line 29:             GridViewRow row = GridView1.Rows[rowIndex];
Line 30:             lbljudul.Text = row.Cells[1].Text;
Line 31:             //lbljudul.Text = (row.FindControl("lbljudul") as Label).Text;