I have tried your sample code.It is throwing the following exception

System.Windows.Markup.XamlParseException: Set property '' threw an exception. [Line: 12 Position: 61] ---> System.NotSupportedException: Cannot set read-only property ''.
   at MS.Internal.XamlMemberInfo.SetValue(Object target, Object value)
   at MS.Internal.XamlManagedRuntimeRPInvokes.SetValue(XamlTypeToken inType, XamlQualifiedObject& inObj, XamlPropertyToken inProperty, XamlQualifiedObject& inValue)
   --- End of inner exception stack trace ---
   at System.Windows.Application.LoadComponent(Object component, Uri resourceLocator)
   at XamTimelineTitleToolTip.MainPage.InitializeComponent()
   at XamTimelineTitleToolTip.MainPage..ctor()}