In my project, I have a MasterPage.
In MasterPage I have the following code :

<%@ Master Language="C#" AutoEventWireup="true" CodeBehind="Master.master.cs" Inherits="KMS.WebUI.Master" %>

<%@ Register Assembly="Ext.Net" Namespace="Ext.Net" TagPrefix="ext" %>
<%@ Register TagPrefix="uc2" TagName="LoginView" Src="UserControls/LoginView.ascx" %>
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
<html xmlns="http://www.w3.org/1999/xhtml">
<head id="Head1" runat="server">
    <title></title>
    <ext:ResourceManager ID="ResourceManager1" runat="server" Locale="fa-ir" AutoDataBind="True" />
    <asp:ContentPlaceHolder ID="head" runat="server">
    </asp:ContentPlaceHolder>
</head>
<body id="Mds_MasterBody">
    <form id="form1" runat="server">
    <!-- *********************************************************************** -->
		....
    <!-- *********************************************************************** -->
    </form>
</body>
</html>
Also, I have some Nested Web Forms from master.master like the following :

<%@ Page Title="مدیریت اعضاء" Language="C#" MasterPageFile="~/Master.master" AutoEventWireup="true"
    CodeBehind="MembersListPage.aspx.cs" Inherits="KMS.WebUI.Pages.Member.MemberRegistration.MembersListPage" %>

<%@ Register Assembly="Ext.Net" Namespace="Ext.Net" TagPrefix="ext" %>
<asp:Content ID="Content1" ContentPlaceHolderID="head" runat="server">
    <style type="text/css">
        .GridPanelMembersRowRed
        {
            background: #FEE9E9;
        }
    </style>
    <script type="text/javascript">
        function GridPanelMembers_ChangeRowColor(record) {
            if (record.data.IsLock) {
                return "GridPanelMembersRowRed";
            }
            else {
                return "";
            }
        }

        function RedirectMessage(url) {
            Ext.Msg.show({
                title: 'در حال انتقال',
                msg: 'در حال انتقال به صفحه `ویرایش کاربر` هستید ، کمی صبر کنید',
                buttons: Ext.Msg.NONE,
                icon: Ext.MessageBox.INFO
            });
            window.location = url;
        }
        
        function EditUser() {
            var gridPanel = Ext.getCmp('<%= GridPanelMembers.ClientID %>');
            var id = gridPanel.getSelectionModel().getSelected().data.Id;
            var url = "<% = ResolveClientUrl("~/Pages/Member/MemberRegistration/MemberRegistrationPage.aspx") %>" + "?id=" + id;
            RedirectMessage(url);
        }
    </script>
</asp:Content>
<asp:Content ID="Content2" ContentPlaceHolderID="ContentPlaceHolder1" runat="server">
    <ext:Store ID="Store1" runat="server" OnRefreshData="Store1_RefreshData">
        <Reader>
            <ext:JsonReader IDProperty="Id">
                <Fields>
                    <ext:RecordField Name="Id" Type="Int" />
                    <ext:RecordField Name="FirstName" />
                    <ext:RecordField Name="LastName" />
                    <ext:RecordField Name="JobTitle" />
                    <ext:RecordField Name="SSN" />
                    <ext:RecordField Name="UserName" />
                    <ext:RecordField Name="MemberStatusFarsiName" />
                    <ext:RecordField Name="MemberRoleName" />
                    <ext:RecordField Name="RegisterDate" />
                    <ext:RecordField Name="IsLock" />
                </Fields>
            </ext:JsonReader>
        </Reader>
    </ext:Store>
    <!-- ****************************************************************************************** -->
    <ext:GridPanel ID="GridPanelMembers" runat="server" Height="500" StoreID="Store1" ClientIDMode="Static"
        AutoExpandColumn="FirstName" Header="true" Title=" لیست اعضا " Icon="User" StripeRows="True">
        <TopBar>
            <ext:Toolbar ID="Toolbar1" runat="server" >
                <Items>
                    <ext:Button ID="ButtonNewUser" runat="server" Text=" ثبت کاربر جدید " Icon="UserAdd">
                        <Listeners>
                            <Click Handler="window.location='MemberRegistrationPage.aspx'"></Click>
                        </Listeners>
                    </ext:Button>
                    <ext:Button ID="ButtonUnlockUser" runat="server" Text=" فعال کردن کاربر " Icon="UserBrown">
                        <DirectEvents>
                            <Click OnEvent="ButtonUnlockUser_Click"  Buffer="300">
                                <EventMask ShowMask="true" Target="CustomTarget"  CustomTarget="={#{GridPanelMembers}.body}" />
                                <Confirmation ConfirmRequest="true" Title="تاییدیه" Message="از فعال کردن کاربر انتخاب شده ، مطمئن هستید" />
                                <ExtraParams>
                                    <ext:Parameter Name="id"  Value="#{GridPanelMembers}.getSelectionModel().getSelected().data.Id"
                                        Mode="Raw" />
                                    <ext:Parameter Name="username"  Value="#{GridPanelMembers}.getSelectionModel().getSelected().data.UserName"
                                        Mode="Raw" />
                                </ExtraParams>
                            </Click>
                        </DirectEvents>
                    </ext:Button>
                    <ext:Button ID="ButtonLockUser" runat="server" Text=" غیر فعال کردن کاربر " Icon="UserRed">
                        <DirectEvents>
                            <Click OnEvent="ButtonLockUser_Click" >
                                <EventMask ShowMask="true" Target="CustomTarget"  CustomTarget="={#{GridPanelMembers}.body}" />
                                <Confirmation ConfirmRequest="true" Title="تاییدیه" Message="از غیر فعال کردن کاربر انتخاب شده ، مطمئن هستید" />
                                <ExtraParams>
                                    <ext:Parameter Name="id" Value="#{GridPanelMembers}.getSelectionModel().getSelected().data.Id"
                                        Mode="Raw" />
                                    <ext:Parameter Name="username" Value="#{GridPanelMembers}.getSelectionModel().getSelected().data.UserName" Mode="Raw" />
                                </ExtraParams>
                            </Click>
                        </DirectEvents>
                    </ext:Button>
                    <ext:Button ID="ButtonEditUser" runat="server" Text=" ویرایش مشخصات کاربر " Icon="UserEdit">
                        <Listeners>
                            <Click Fn="EditUser"></Click>
                        </Listeners>
                    </ext:Button>
                </Items>
            </ext:Toolbar>
        </TopBar>
        <View>
            <ext:GridView ID="GridView1" runat="server">
                <GetRowClass Fn="GridPanelMembers_ChangeRowColor" />
            </ext:GridView>
        </View>
        <ColumnModel runat="server">
            <Columns>
                <ext:RowNumbererColumn />
                <ext:Column ColumnID="IsLock" DataIndex="IsLock" Hidden="true" />
                <ext:Column ColumnID="FirstName" Header="نام" DataIndex="FirstName" Align="Right"
                    Width="50" />
                <ext:Column ColumnID="LastName" Header="نام خانوادگی" DataIndex="LastName" Align="Right"
                    Width="100" />
                <ext:Column ColumnID="JobTitle" Header="واحد سازمانی" DataIndex="JobTitle" Align="Right"
                    Width="100" />
                <ext:Column ColumnID="SSN" Header="کد ملی" DataIndex="SSN" Align="Right" Width="80" />
                <ext:Column ColumnID="UserName" Header="نام کاربری" DataIndex="UserName" Align="Right"
                    Width="80" />
                <ext:Column ColumnID="MemberRoleName" Header="نقش" DataIndex="MemberRoleName" Align="Right"
                    Width="120" />
                <ext:Column ColumnID="MemberStatusFarsiName" Header="وضعیت ثبت نام" DataIndex="MemberStatusFarsiName"
                    Align="Right" Width="100" />
                <ext:Column ColumnID="MemberStatusFarsiName" Header="تاریخ ثبت" DataIndex="RegisterDate"
                    Align="Center" Width="100" Locked="true" />
            </Columns>
        </ColumnModel>
        <SelectionModel>
            <ext:RowSelectionModel ID="RowSelectionModel1" runat="server" SingleSelect="true" />
        </SelectionModel>
        <BottomBar>
            <ext:PagingToolbar ID="PagingToolBar1" runat="server" PageSize="50" StoreID="Store1"
                StyleSpec="direction:ltr;" EmptyMsg="&nbsp;" DisplayMsg="&nbsp;" BeforePageText="صفحه"
                HideRefresh="true" />
        </BottomBar>
    </ext:GridPanel>
</asp:Content>
Everything is OK, but if I use ExtNet.Msg.Alert("Title", "Message"); in code behind
I give the following error :

c is undefined
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Could you please guide me ?