Getting Error '0.cells' is null or not an object, when menu is inside of UpdatePanel

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Keep menu outside of update panel and configure the trigger property as the following example code.

<asp:UpdatePanel ID="UpdatePanel2" runat="server">
<ContentTemplate>
</ContentTemplate>
<Triggers>
<asp:AsyncPostBackTrigger ControlID="Menu1" EventName="MenuItemClick" />
</Triggers>
</asp:UpdatePanel>

<asp:Menu ID="Menu1" runat="server" BackColor="#F7F6F3" DynamicHorizontalOffset="2"
Font-Names="Verdana" Font-Size="Small" ForeColor="#7C6F57" OnMenuItemClick="Menu1_MenuItemClick"
StaticSubMenuIndent="10px" Width="150px">
<StaticMenuItemStyle HorizontalPadding="5px" VerticalPadding="2px" />
<DynamicHoverStyle BackColor="#7C6F57" ForeColor="White" />
<DynamicMenuStyle BackColor="#F7F6F3" />
<StaticSelectedStyle BackColor="#5D7B9D" />
<DynamicSelectedStyle BackColor="#5D7B9D" />
<DynamicMenuItemStyle HorizontalPadding="5px" VerticalPadding="2px" />
<Items>
<asp:MenuItem Text="Save" Value="Save"></asp:MenuItem>
<asp:MenuItem Text="New" Value="New"></asp:MenuItem>
</Items>