system.form exeception data gridview combobox cell while inserting data to data grid

[jaseem0712]

I have a data grid view,,in third column of data grid view is combox box,,am filling thise

 Dim c As DataGridViewComboBoxColumn
            Dim cmd2 As New SqlCommand("select desigID,DName from Designation_tbl where deleted=0", con.connect)
            cmd2.CommandType = CommandType.Text
            Dim objdataadapter As New SqlDataAdapter(cmd2)
            Dim results As New DataSet
 
            objdataadapter.Fill(results, "DesignationMaster_tbl")
            c = DGVEmployee.Columns(3)
            c.DataSource = results.Tables("DesignationMaster_tbl")
            c.ValueMember = "desigID"
            c.DisplayMember = "DName"
            con.disconnect()

after that i want to fill my data grid view..
so i manually created all columns..
then i wrote code like this:

Dim Ecode As String
            Dim Ename As String
            Dim email As String
            Dim design As String
    Dim cmd As New SqlCommand("  select eCode,eName,desigID from EmployeeMaster_tbl", con.connect)
     dr = cmd.ExecuteReader
            While dr.Read
                If dr("eCode") Is DBNull.Value Then
                    Ecode = ""
                Else
                    Ecode = dr("eCode")
                End If
                If dr("eName") Is DBNull.Value Then
                    Ename = ""
 
                Else
                    Ename = dr("eName")
 
                End If
                If dr("desigID") Is DBNull.Value Then
                    design = ""
                Else
                    design = dr("desigID")
                    desgnationname = RecordID_String("DName", "Designation_tbl", "desigID", design)
 
                End If
     Dim row0 As String() = {Ecode, Ename, email, desgnationname}
    DGVEmployee.Rows.Add(row0)
      End While

but am getting error: system.form exeception data gridview combobox cell