对象引用未设置为具有母版页的对象错误的实例

Question

获取错误当我尝试发布到墙上时，对象引用未设置为对象的实例。

Site.master.cs

public FacebookSession CurrentSession
{
    get { return (new CanvasAuthorizer()).Session; }
}
protected void Page_Load(object sender, EventArgs e)
{
    var auth = new CanvasAuthorizer { Perms = "email,read_stream,publish_stream,offline_access" };`

    if (auth.Authorize())
    {
        ShowFacebookContent();
    }
}
private void ShowFacebookContent()
{
    var fb = new FacebookClient(this.CurrentSession.AccessToken);
    dynamic myInfo = fb.Get("me");
    lblName.Text = myInfo.name;
    pnlHello.Visible = true;
}
}

Default.aspx.cs

SiteMaster myMasterPage;

protected void Page_Load(object sender, EventArgs e)
{
    myMasterPage = Page.Master as SiteMaster;
}
public void LinkButton1_Click(object sender, EventArgs e)
{
    var fb = new FacebookClient(myMasterPage.CurrentSession.AccessToken);

    dynamic feedparameters = new ExpandoObject();
    feedparameters.message = (message_txt.Text == null ? " " : message_txt.Text);
    feedparameters.user_message_prompt = " ";

    /*Dictionary<string, object> feedparameters = new Dictionary<string, object>();

    feedparameters.Add("message", "Testing Application");
    feedparameters.Add("user_message_prompt", "Post To Your Wall");
    feedparameters.Add("display", "iframe");*/

    dynamic result = fb.Post("me/feed", feedparameters);
}
}

单击LinkBut​​ton时会发生错误。

任何帮助将不胜感激。