Web Forms :: Master Page Control In Child Page Error "Object Reference Not Set To An Instance Of An Object"

i have create dynamic menu control , i have master page and login page , once the user login , i want to show the dynamic menu in my master page. below i try to call masterpage control in my child page ,but i throughs the error

Object reference not set to an instance of an object.
my master page code
public partial class MasterPage : System.Web.UI.MasterPage
{
protected void Page_Load(object sender, EventArgs e)
{
}
public string MyProperty
{
set
{
// lblUser.Text = value;
lblUser.Text = value; // here i got the error
}
}
}

My child page code

protected void btnwork_Click(object sender, EventArgs e)
{
((MasterPage)this.Master).MyProperty = "Text changed from Sub Page";
}