trying to avoid System.ArgumentOutOfRangeException dropdown list in a datagrid

hello dear experts,
i have  a problem with a dropdown list in a datagrid, basically i am trying to avoid the:
System.ArgumentOutOfRangeException: Specified argument was out of the range of valid values. Parameter name: value
Source Error:
Line 67:                         
Line 68:                         BOMDataGrid.DataSource = DS.Tables["BOM"];
Line 69:                         BOMDataGrid.DataBind();      ----> marked red

after readings some replies here i added the GetSelectedValue function.

<asp:datagrid id="BOMDataGrid" runat="server" AutoGenerateColumns="False" OnEditCommand="Edit"             OnCancelCommand="Cancel" OnUpdateCommand="Update" OnDeleteCommand="Delete">
......
         <asp:DropDownList id="MaterialDDL" runat="server" DataSource='<%# MaterialDT%>' DataTextField="Material" DataValueField="ID" SelectedValue='<%# GetSelectedValue(DataBinder.Eval(Container, "DataItem.MaterialID"))%>'>
         </asp:DropDownList>

public String GetSelectedValue(object selVal)
{      
     if (selVal.Equals("") || selVal==null)
         return null;
     else
         return selVal.ToString();                              
}

i have a:
public DataTable MaterialDT; in the .cs

and am calling GetData() to populate and bind the datagrid and to poulate the datatable for the dropdownlist:MaterialDT.


            void GetData()
            {                  
                        SqlCommand dataCommand = new SqlCommand();
                        SqlConnection dataConnection = new SqlConnection();
                        dataConnection.ConnectionString = dbConn;
                        dataCommand.Connection = dataConnection;                  
                        
                        dataCommand.CommandText = "GetMaterials";
                        dataCommand.CommandType = System.Data.CommandType.StoredProcedure;      
                  
                        SqlDataAdapter MaterialDA = new SqlDataAdapter(dataCommand);
                        MaterialDA.Fill(DS,"Material");

                        MaterialDT = DS.Tables["Material"];
                        
                        SqlParameter reportIDprm;
                        reportIDprm= new SqlParameter("@reportID",1);                  
                        dataCommand.Parameters.Add(reportIDprm);

                        dataCommand.CommandText = "GetBOM";
                        dataCommand.CommandType = System.Data.CommandType.StoredProcedure;      

                        BOMDA = new SqlDataAdapter(dataCommand);
                        BOMDA.Fill(DS,"BOM");
                        
                        BOMDataGrid.DataSource = DS.Tables["BOM"];
                        BOMDataGrid.DataBind();      <-----------------------error here                  
                  /*
                  }
                  catch(ArgumentOutOfRangeException OutOfRange)
                  {
                        Label1.Text = OutOfRange.Message;
                  }
                  catch(Exception e)
                  {
                        Label1.Text = e.Message;
                  }
                                         */
            }
but am still getting the error on pressing the edit button in the datagrid on items that were added with the add material button:
which only has this at its end:

BOMDataGrid.EditItemIndex = -1;
GetData();      

hope you might help, 10x in advance.