Hello Sathya,
Somewhere in your code, you are assigning a null value to another variable. You need to add a try-catch block there. somewhat like this
try {
TextBox3.text = DropDownList3.Items.FindByText(TextBox3.Text).Value
} catch (NullReferenceException ex) {
TextBox3.text = "(none)";
}
Thanks!! Hope it help.