using javascript code to validate the checkbox in Grid view

hi,

I think This error due to find some null object and you cannot access null object property.

so try..
function validateCheckBoxes() {
    var isValid = false;
    var gridView = document.getElementById('<%= GrdEmp.ClientID %>');
    for (var i = 1; i < gridView.rows.length; i++) {
        var inputs = gridView.rows[i].getElementsByTagName('input');
        if (inputs != null && inputs[0] != null) {
            if (inputs[0].type == "checkbox") {
                if (inputs[0].checked) {
                    isValid = true;
                    return true;
                }
            }
        }
    }
    alert("Please select atleast one checkbox");
    return false;
}

you can also use Jquery for same.

hope this will help you.