First you see the .aspx.Designer.cs Page.
I think there is not create the textcontrol name. so, it is not working..
So, you manually write the code to designer.Cs page.
then it will work.. try this way...
/// <summary>
/// txtSenderId control.
/// </summary>
/// <remarks>
/// Auto-generated field.
/// To modify move field declaration from designer file to code-behind file.
/// </remarks>
protected global::System.Web.UI.WebControls.TextBox txtSenderId;