C# - How To Handle The Click Event Of A Button In A Custom ITemplate Control

I've created a custom control in ASP.NET for showing a pop-up message box. In my MessageBox class, I have a Content property as ITemplate like the following:

[PersistenceMode(PersistenceMode.InnerProperty)]
[TemplateContainer(typeof(MessageBoxContent))]
[TemplateInstance(TemplateInstance.Single)]
public ITemplate Content { get; set; }

I used my custom control in my page like this:

<cc1:MessageBox ID="MessageBox1" runat="server">
<Content>
<asp:Button Text="Save" runat="server" ID="B_Save" />
</Content>
</cc1:MessageBox>

Even I set the the Content property's TemplateInstance to Single, I still can't have access to the Button control.

All I want to do is to handle the click event of the button. When I assign an event handler to the button control and run the project, it throws a NullReferenceException.