Error : Input string was not in a correct format.

Code:
public partial class frmDealerForm : Form
    {
        DataTable t;
        DataRow r;
        string id;
        private void btnNew_Click(object sender, EventArgs e)
        {
            txtdealerID.ResetText();
            txtname.ResetText();
            txtaddress.ResetText();
            txtphoneno.ResetText();
...