When document fragment containing consecutive spans with same style is inserted with RadRichTextBox.InsertFragment method, and the document is with specific structure (which is not exactly known), NullReferenceException in HierarchicalIndex.GetBoxByHierarchicalIndex could be thrown when text is typed in specific places in the document.

Workaround: Ensure that such spans are merged before creating the fragment:

var document = xamlProvider.Import(xaml);
document.MergeSpansWithSameStyles();
var fragment = new DocumentFragment(document);
rtb.InsertFragment(fragment);

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Sample call stack:

System.NullReferenceException
 at Telerik.Windows.Documents.HierarchicalIndex.GetBoxByHierarchicalIndex(DocumentLayoutBox documentBox, HierarchicalIndex hierarchicalIndex)
 at Telerik.Windows.Documents.DocumentPosition.RestorePositionFromBoxIndex(Nullable`1 raiseEvent)
 at Telerik.Windows.Documents.Model.RadDocumentEditor.InsertTextInternal(String text, Span currentSpanStyle, Boolean explicitAcceptsReturn)
 at Telerik.Windows.Documents.Model.RadDocumentEditor.InsertFromUI(String text, Span currentSpanStyle, Boolean acceptsReturn)
 at Telerik.Windows.Controls.RadRichTextBox.Telerik.Windows.Documents.UI.ITextInputCommandsHandler.InsertText(String text)
 at Telerik.Windows.Documents.UI.CaretTextInputHandler.InsertText(String text)
 at Telerik.Windows.Documents.UI.CaretTextInputHandler.HandleTextInsertedWithoutIme(RadDocument document, String text)
 at Telerik.Windows.Documents.UI.CaretTextInputHandler.CaretUI_TextInserted(Object sender, TextInsertedEventArgs e)
 at Telerik.Windows.Documents.UI.Caret.OnTextInserted(Object sender, TextInsertedEventArgs e)
 at Telerik.Windows.Documents.UI.Caret.OnTextChanged()
 at Telerik.Windows.Documents.UI.Caret.Caret_TextChanged(Object sender, TextChangedEventArgs e)

...

Fix available in LIB Version 2018.2.813.