object reference not set to an instance of object c#

Question

I work in windows form c# vs 2015 i get error i need to solve it

 

but i dont know how to solve it

 

i write function in textbox changed event but when i start writing in textbox

 

get error object reference not set to an instance of object .

 

so that why this problem happen and how to solve it .

 

in textbox changed event

 

private void textBox3_TextChanged(object sender, EventArgs e)
       {
           try
           {


                   MatrixClass matrix = new MatrixClass();
                   string value1 = matrix.GetAtcCode(textBox3.Text);
                   textBox1.Text = value1;


           }
           catch (Exception ex)
           {
               MessageBox.Show(ex.Message);
           }
       }

 

public string GetAtcCode(string GenericName)
       {
           SqlConnection con = new SqlConnection(connection);

           con.Open();

           SqlCommand comm = new SqlCommand("ATC_Code_ByGeneric_H", con);

           comm.CommandType = CommandType.StoredProcedure;

           comm.Parameters.Add(new SqlParameter("@Lvl_Name", GenericName));

           string ret = comm.ExecuteScalar().ToString();
           con.Close();

           return ret;

       }

 stored procedure 

ALTER PROCEDURE [dbo].[ATC_Code_ByGeneric_H]
    @Lvl_Name nvarchar(500)
AS
    SELECT     Tbl_Lvl1.Code + Tbl_Lvl2.Code + Tbl_Lvl3.Code + Tbl_Lvl4.Code + Tbl_Lvl5.Code AS ATC_Code
    FROM         Tbl_Lvl1
                 INNER JOIN
                 Tbl_Lvl2
                 ON Tbl_Lvl1.S_ID = Tbl_Lvl2.UpLvl_ID
                 INNER JOIN
                 Tbl_Lvl3
                 ON Tbl_Lvl2.S_ID = Tbl_Lvl3.UpLvl_ID
                 INNER JOIN
                 Tbl_Lvl4
                 ON Tbl_Lvl3.S_ID = Tbl_Lvl4.UpLvl_ID
                 INNER JOIN
                 Tbl_Lvl5
                 ON Tbl_Lvl4.S_ID = Tbl_Lvl5.UpLvl_ID
    WHERE     (Tbl_Lvl5.Lvl_Name = @Lvl_Name) and Tbl_Lvl5.human=1
    RETURN