NullReferenceException is thrown when a hyperlink is removed from the document while the user is hovering it with the mouse. Such cases are when hyperlink is removed from the context menu, or when TOC field containing hyperlinks is updated from the context menu. The exception is with the following call stack: ErrorMessage: Object reference not set to an instance of an object. StackTrace: at Telerik.Windows.Documents.UI.UIProviders.HyperlinkUIProviderBase.UIElement_MouseEnter(Object sender, MouseEventArgs e) at System.Windows.RoutedEventArgs.InvokeHandler(Delegate handler, Object target) ... If the mouse hovers the hyperlink a moment later, exception is not observed. The situation when the mouse is over a hyperlink is not so uncommon: - When the user operates hyperlink context menu with the keyboard and removes the hyperlink - when there are multiple items in the ToC, there is great probability for the mouse to be over a hyperlink which is just below the Update Field button in the context menu - and when the context menu disappears, the mouse gets over the hyperlink. Fix available in LIB Version 2017.2.529.