Add new row to gridview not working inside repeater control

Hello, I'm using Gridview inside a repeater controlto select items for billing.

but when I try to add new row it says:

"Object reference not set to an instance of an object."

Here is my c# code for the same:

protected void Page_Load(object sender, EventArgs e)
{
if (!this.IsPostBack)
{
this.getTableDetails();
}
}
protected void getTableDetails()
{
DataTable dt = new DataTable();
SqlDataAdapter sda = new SqlDataAdapter("Select TableName From TableMaster where TableStatus='0'", constring);
sda.Fill(dt);
rptrTables.DataSource = dt;
rptrTables.DataBind();
}
protected void rptrTables_ItemDataBound(object sender, RepeaterItemEventArgs e)
{
GridView grdItemDetails = e.Item.FindControl("grdOrder") as GridView;
DataTable dt = new DataTable();
DataRow dr = null;
dt.Columns.Add(new DataColumn("RowNumber", typeof(string)));
dt.Columns.Add(new DataColumn("ItemName", typeof(string)));
dt.Columns.Add(new DataColumn("UnitPrice", typeof(string)));
dt.Columns.Add(new DataColumn("Quantity", typeof(string)));
dt.Columns.Add(new DataColumn("LineTotal", typeof(string)));
dr = dt.NewRow();
dr["RowNumber"] = 1;
dr["ItemName"] = string.Empty;
dr["UnitPrice"] = string.Empty;
dr["Quantity"] = string.Empty;
dr["LineTotal"] = string.Empty;
dt.Rows.Add(dr);
ViewState["CurrentTable"] = dt;
grdItemDetails.DataSource = dt;
grdItemDetails.DataBind();
}
private void AddNewRowToGrid()
{
int x = 0;
foreach (RepeaterItem rptrItems in rptrTables.Items)
{
GridView gvItems = (GridView)rptrItems.FindControl("grdOrder");
int rowIndex = 0;
if (ViewState["CurrentTable"] != null)
{
DataTable dtCurrentTable = (DataTable)ViewState["CurrentTable"];
DataTable dt = dtCurrentTable.Copy();
DataRow drCurrentRow = dt.NewRow();
if (dt.Rows.Count > 0)
{
for (int i = 1; i <= dt.Rows.Count; i++)
{
DropDownList box1 = (DropDownList)gvItems.Rows[rowIndex].Cells[0].FindControl("drpItemname");
TextBox box2 = (TextBox)gvItems.Rows[rowIndex].Cells[0].FindControl("lblUnitPrice");
TextBox box3 = (TextBox)gvItems.Rows[rowIndex].Cells[0].FindControl("lblQuantity");
TextBox box4 = (TextBox)gvItems.Rows[rowIndex].Cells[0].FindControl("lblLineTotal");
drCurrentRow = dt.NewRow();
drCurrentRow["RowNumber"] = i + 1;
dt.Rows[i - 1]["ItemName"] = box1.Text;
dt.Rows[i - 1]["UnitPrice"] = box2.Text;
dt.Rows[i - 1]["Quantity"] = box3.Text;
dt.Rows[i - 1]["LineTotal"] = box4.Text;
rowIndex++;
}
dt.Rows.Add(drCurrentRow);
if (x == rptrTables.Items.Count - 1)
{
ViewState["CurrentTable"] = dt;
}
gvItems.DataSource = dt;
gvItems.DataBind();
}
//SetPreviousData(gvItem);
}
else
{
Response.Write("ViewState is null");
}
x++;
}
}
protected void AddItem(object sender, EventArgs e)
{
AddNewRowToGrid();
}
private void SetPreviousData()
{
GridView gvItems = (GridView)rptrTables.Items[0].FindControl("grdOrder");
int rowIndex = 0;
if (ViewState["CurrentTable"] != null)
{
DataTable dt = (DataTable)ViewState["CurrentTable"];
if (dt.Rows.Count > 0)
{
for (int i = 0; i < dt.Rows.Count; i++)
{
DropDownList box1 = (DropDownList)gvItems.Rows[rowIndex].Cells[0].FindControl("drpItemname");
TextBox box2 = (TextBox)gvItems.Rows[rowIndex].Cells[0].FindControl("lblUnitPrice");
TextBox box3 = (TextBox)gvItems.Rows[rowIndex].Cells[0].FindControl("lblQuantity");
TextBox box4 = (TextBox)gvItems.Rows[rowIndex].Cells[0].FindControl("lblLineTotal");
box1.Text = dt.Rows[i]["ItemName"].ToString();
box2.Text = dt.Rows[i]["UnitPrice"].ToString();
box3.Text = dt.Rows[i]["Quantity"].ToString();
box4.Text = dt.Rows[i]["LineTotal"].ToString();
rowIndex++;
//if (!string.IsNullOrEmpty(box5.Text) && !string.IsNullOrEmpty(box5.Text))
//{
// box5.Text = dt.Rows[i]["Total"].ToString();
//}
//else
//{
// box5.Text = "0";
//}
}
}
}
}
protected void GetItemDetails(object sender, EventArgs e)
{
foreach (RepeaterItem rptrItems in rptrTables.Items)
{
GridView gvItems = (GridView)rptrItems.FindControl("grdOrder");
//GridView gvItems = (GridView)rptrTables.Items[0].FindControl("grdOrder");
foreach (GridViewRow row in gvItems.Rows)
{
DropDownList ddl = sender as DropDownList;
Control ctrl = row.FindControl("drpItemname") as DropDownList;
if (ctrl != null)
{
DropDownList ddl1 = (DropDownList)ctrl;
if (ddl.ClientID == ddl1.ClientID)
{
TextBox UnitPrice = row.FindControl("lblUnitPrice") as TextBox;
TextBox QTTY = row.FindControl("lblQuantity") as TextBox;
TextBox UPrice = row.FindControl("lblUnitPrice") as TextBox;
TextBox LINETOTAL = row.FindControl("lblLineTotal") as TextBox;
SqlConnection conn = new SqlConnection(constring);
conn.Open();
if (ctrl != null)
{
if ((ddl1.ID == ddl.ID) && (ddl1.SelectedIndex == ddl.SelectedIndex))
{
string str = "select * from ItemMaster where ItemName='" + ddl1.SelectedItem.ToString() + "'";
SqlCommand com = new SqlCommand(str, conn);
SqlDataReader reader = com.ExecuteReader();
while (reader.Read())
{
UnitPrice.Text = reader["UnitPrice"].ToString();
//decimal totamt = Convert.ToDecimal(QTTY.Text) * Convert.ToDecimal(UnitPrice.Text);
}
reader.Close();
conn.Close();
}
else
{
ScriptManager.RegisterStartupScript(this, this.GetType(), "swal", "swal('Item already selected, you can increase the quantity instead!', 'Happy Cakes, Vijayapura', 'warning');", true);
}
}
}
}
}
}
}
protected void Gridview1_RowDeleting1(object sender, GridViewDeleteEventArgs e)
{
DataTable dt = (DataTable)ViewState["CurrentTable"];
GridView gvItems = (GridView)rptrTables.Items[0].FindControl("grdOrder");
//txtAmtPayable.Text = (Convert.ToDecimal(txtAmtPayable.Text) - Convert.ToDecimal(((TextBox)gvItems.Rows[e.RowIndex].FindControl("txtTotalPrice")).Text)).ToString("#0.00");
dt.Rows.RemoveAt(e.RowIndex);
gvItems.DataSource = dt;
gvItems.DataBind();
SetPreviousData();
if (gvItems.Rows.Count <= 0)
{
ScriptManager.RegisterStartupScript(this, this.GetType(), "swal", "swal('This is the only item in the list!', 'Royal Riders, Vijayapura', 'success');", true);
SetInitialRow();
/*txtAmtPaid.Text = "0";
txtAmtPayable.Text = "0";
txtBalAmt.Text = "0";
txtcpan.Text = "NA";
txtgst.Text = "NA";*/
}
}

And below is my ASPX Markup:

<div class="row">
<asp:Repeater ID="rptrTables" runat="server" OnItemDataBound="rptrTables_ItemDataBound">
<ItemTemplate>
<div class="col-md-6">
<div class="card card-info">
<div class="card-header with-border">
<h3 class="card-title"><asp:Label ID="lblTableNumber" runat="server" Text='<%# Eval("TableName") %>'>asp:Label>h3>
<div class="card-tools">
<button type="button" class="btn btn-tool" data-widget="collapse"><i class="fa fa-minus">i>button>
div>
div>
<div class="card-body">
<div class="row">
<div class="col-md-12">
<div class="form-group">
<asp:GridView ID="grdOrder" runat="server" AutoGenerateColumns="false" AllowPaging="true" CssClass="table table-bordered table-hover table-responsive" GridLines="None" PageSize="10" OnRowDataBound="gvRowDataBound" OnRowDeleting="Gridview1_RowDeleting1" ShowFooter="true">
<Columns>
<asp:CommandField ShowDeleteButton="true" ControlStyle-CssClass="btn btn-danger fa fa-trash" DeleteText="" HeaderText="Remove" />
<%--<asp:BoundField DataField="RowNumber" HeaderText="Sl. No." />--%>
<asp:TemplateField HeaderText="Item Name">
<ItemTemplate>
<asp:DropDownList ID="drpItemname" runat="server" CssClass="form-control select2" OnSelectedIndexChanged="GetItemDetails" AutoPostBack="true">asp:DropDownList>
ItemTemplate>
asp:TemplateField>
<asp:TemplateField HeaderText="Price">
<ItemTemplate>
<asp:TextBox ID="lblUnitPrice" runat="server" CssClass="form-control" Text='<%# Eval("UnitPrice")%>'>asp:TextBox>
ItemTemplate>
asp:TemplateField>
<asp:TemplateField HeaderText="Qty">
<ItemTemplate>
<asp:TextBox ID="lblQuantity" runat="server" TextMode="Number" CssClass="form-control" Text="1">asp:TextBox>
ItemTemplate>
asp:TemplateField>
<asp:TemplateField HeaderText="Line Total">
<ItemTemplate>
<asp:TextBox ID="lblLineTotal" runat="server" CssClass="form-control" Text="0">asp:TextBox>
ItemTemplate>
<FooterStyle HorizontalAlign="Right" />
<FooterTemplate>
<asp:Button ID="ButtonAdd" runat="server" CssClass="btn btn-primary" Text="Add" OnClick="AddItem" CausesValidation="False" OnCommand="ButtonAdd_Command" />
FooterTemplate>
asp:TemplateField>
Columns>
asp:GridView>
div>
div>
div>
div>
<div class="card-footer">
<div class="pull-right">
<asp:Button ID="btnSubmit" runat="server" CausesValidation="false" TabIndex="2" CssClass="btn btn-primary" Text="Print KOT" />
<asp:Button ID="Button1" runat="server" CausesValidation="false" TabIndex="2" CssClass="btn btn-primary" Text="Print Final Bill" />
<asp:Button ID="Button2" runat="server" CausesValidation="false" TabIndex="2" CssClass="btn btn-primary" Text="Complete Order" />
div>
div>
div>
div>
ItemTemplate>
asp:Repeater>
div>

Please help!