Showing a document that contains a Table object without any TableRow inside leads to NullReferenceException.

Workaround: After importing the document, remove the tables that don't have content:

foreach (var table in document.EnumerateChildrenOfType<Table>())
{
    if (table.Rows.Count < 1)
    {
        table.Parent.Children.Remove(table);
    }
}

To reproduce:

1. Insert content spanning 3 pages
2. Ensure a single page is visualized in the viewport 
3. Start selecting from the beginning of the content till the end by dragging the mouse
4. Press Delete

The issue is a regression introduced in R3 2021 SP1.

The parent of an already removed span cannot be found and a NullReferenceException is thrown while performing Undo. Also, multiple asserts are failing in Debug mode. The stack trace is:

UnhandledException: Object reference not set to an instance of an object.
UnhandledException:   at Telerik.Windows.Documents.Layout.LayoutElement.SetParent(LayoutElement newParent)
   at Telerik.Windows.Documents.Layout.LayoutBox.EnsureParent()
   at Telerik.Windows.Documents.Layout.LayoutBox.get_Parent()
   at Telerik.Windows.Documents.Layout.LayoutBox.get_ValidParent()
   at Telerik.Windows.Documents.WordPositionHandler.GetParentHandler()
   at Telerik.Windows.Documents.DocumentPosition.GetCurrentSectionBox()
   at Telerik.Windows.Documents.UI.DocumentPrintLayoutPresenter.GetCurrentPage()
   at Telerik.Windows.Documents.UI.DocumentPrintLayoutPresenter.MeasureOverride(Size availableSize)
   at System.Windows.FrameworkElement.MeasureCore(Size availableSize)

Working with PasteOptionsPopup may cause an exception in LayoutElement.SetParent. Very difficult to reproduce, having no consistent steps. Maybe it is related to undo of paste command, but there are no other clues aside from this.