Make Sure that your gridview can accept only numbers you can have a filterextender using ajax and I m sure u will do that what else you can do is to check whether you have a textbox is null or not using the Function given below
if(string.IsNullOrEmpty(((TextBox)Row.FindControl("numTC")).Text))
{}
((TextBox)GridViewname.Rows[e.RowIndex].FindControl("numTC")).Text;
and
use this extender or u can use javascript as well
If it is going inside the if statement that means the value is null
if(!string.IsNullOrEmpty(((TextBox)row.FindControl("numTC")).Text)) {}
I have used ! sign now it will go inside the if statement if there is some value in it.
and try to convert this text into integer using try catch block if u get any exception you can take whatever action you want to.
Let me know if it is complete