private String ConvertNullToEmptyString(DataTable element)
    {
        if (element.Rows.Count > 0) //just add this condition here
        {
            if (!DBNull.Value.Equals(element.Rows[0]["FullName"]))
                return (string)element.Rows[0]["FullName"] + " ";
            else

private DataTable getSomething1(String text){
    DataTable dtb = new DataTable();
    ...
    ...
    return dtb;
}

...

protected void buttonCheck_Click(object sender, ImageClickEventArgs e){
    List<DataTable> dtList = new List<DataTable>(){
        getSomething(item1.Text),
        getSomething2(item2.Text),
        getSomething3(item3.Text),