c# System.NullReferenceException: Object reference not set to an instance of an object

<asp:Literal ID="LtError" runat="server" />
<asp:PlaceHolder ID="PhWrapper" runat="server">
<h2>Create New Ballot</h2>
<asp:MultiView ID="MvNewBallot" runat="server" ActiveViewIndex="0">
    <asp:View ID="ViewForm" runat="server">
        <div class="generic-form-section" style="text-align:center";>
                      
              <strong>Candidats:</strong><br />
               <p><strong>1.</strong><asp:TextBox ID="Cand1" Width="313px" runat="server" /><br />
              Information Link:<asp:TextBox ID="TxtLink1" Width="313px" runat="server" /><br /></p>
             <p>2.</strong><asp:TextBox ID="Cand2" Width="313px" runat="server" /><br />
            Information Link:<asp:TextBox ID="TxtLink2"  Width="313px" runat="server" /><br /></p>
       
             <p><strong>Status: </strong><asp:DropDownList ID="DdlStatus" runat="server">
                    <asp:ListItem Text="Active" Value="Active" />
                    <asp:ListItem Text="Inactive" Value="Inactive" Selected="True" />
              </asp:DropDownList></p>
        </div>   
        <p>
            <asp:LinkButton ID="BtnCreate" runat="server" Text="Create Ballot" CssClass="btn" OnClick="BtnCreate_Click" />
        </p>
    </asp:View>
    <asp:View ID="ViewThanks" runat="server">
        <p>The Ballot has been successfully created!</p>
    </asp:View>
</asp:MultiView>
</asp:PlaceHolder>

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protected void BtnCreate_Click(object sender, EventArgs e)
        {
            try
            {
               
               AddNewResponses();
               MvNewBallot.SetActiveView(ViewThanks);
            }
            catch (Exception ex)
            {
                Response.Write(ex);

                LtError.Text = "An error has occurred while attempting to submit your application. <span style=\"display: none;\">" + ex.ToString() + "</span>";
            }
        }
        private void AddNewResponses()
        {
            CDataAccess da = null;
            using (da = new CDataAccess(_connectionString))
            {
                da.OpenConnection();
                int resseq = 0;

                foreach (Control ct in this.Controls)
                {
                    TextBox tb = ct as TextBox;
                    if (tb.ID.Contains("Cand"))
                    {
                        string rlink = "";
                        if (!string.IsNullOrWhiteSpace(tb.Text))   // (tb.Text.Length > 0) //&
                        {
                                ++resseq;
                                da.SelectParameterizedStoredProcedure("AddNewResponse");
                                da.AddInputObject("ResSeq", System.Data.SqlDbType.Int, resseq);
                                da.AddInputObject("ResText", System.Data.SqlDbType.VarChar, tb.Text);
                                if (tb.ID == "Cand1")
                                {
                                    rlink = TxtLink1.Text;
                                }
                                if (tb.ID == "Cand2")
                                {
                                    rlink = TxtLink2.Text;
                                }
                                
                                if (!String.IsNullOrWhiteSpace(rlink))
                                {
                                    da.AddInputObject("ResLink", System.Data.SqlDbType.VarChar, rlink);
                                }
                                da.ExecuteParameterizedCommand();
                                da.ClearResultsAndParameters();
                                da.CloseConnection();
                                da.Dispose();
                            }
                        }
                    }
                }
            }

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