Solution Page Validation error with "EnableEventValidation=true" Command

Add the below line if grid bind in page load  :--
if (!Page.IsPostBack)
{
DataTable  dt=methodName_that_fill_datatable();
}

add
<%@ Page AutoEventWireup="true" >

and remove<%@ Page EnableEventValidation="true" >

How to convert Null value of database to some value

If you need to change null value of database at the time of  fetching data from database
then use this method ISNULL

SELECT  ISNull(ColumnName,'Your Converted Value ')  FROM TABLENAME