How To Load Default Content On A Master Page

I have the following placeholder in my main master page. I would like to not have to duplicate my news content in several content pages, so I choose to simply not provide this content in them, and the master page should provide its default.

<asp:ContentPlaceHolder ID="SideBarContent" runat="server">
<asp:GridView ID="newsGrid" runat="server" AutoGenerateColumns="false" Width="100%">
<Columns>
<asp:TemplateField>
<ItemTemplate>
</ItemTemplate>
</asp:TemplateField>
</Columns>
</asp:GridView>
</asp:ContentPlaceHolder>

When I try the following code, I get an error because newsGrid is null. I assume I'm doing this at the wrong place in the page lifecycle, but I don't know where the right place is.

protected void Page_Load(object sender, EventArgs e)
{
Page.Header.DataBind();
if (!IsPostBack)
{
newsGrid.DataSource = _newsService.ListActive();
newsGrid.DataBind();
}
}