Please see the attached video.
I've managed to fix this by altering the source code of method ParagraphLayoutBox.MeasureOverride and hope it doesn't cause other problems.
As shown in the attached video.
Btw, I was using the MS Pinyin IME.
NullReferenceException is thrown when the "var" CSS function is used in the HTML on HtmlFormatProvider Import method call.
The exception occurs only for CSS properties processed by RadRichTextBox. For example: "vertical-align", "font-family", "font-weight", "text-decoration-underline", etc.
The following HTML produces such error:
<span style="vertical-align: var(--my-variable);">
</span>
If the span tag doesn't have any content, the error doesn't occur.
The exception stacktrace is:
at Telerik.Windows.Documents.FormatProviders.Html.Import.HtmlDocumentImporter.FillSpanProperties(SpanProperties spanProps, EvaluateProperty evalFunction, IEnumerable`1 disregarderdDefaultStyleBagPropertiesCollection)
at Telerik.Windows.Documents.FormatProviders.Html.Import.HtmlDocumentImporter.CreateSpanFromTextNode(INode childNode, Paragraph paragraph, IEnumerable`1 disregarderdDefaultStyleBagPropertiesCollection)
at Telerik.Windows.Documents.FormatProviders.Html.Import.HtmlDocumentImporter.ProcessContentNode(INode node)
at Telerik.Windows.Documents.FormatProviders.Html.Import.HtmlDocumentImporter.<>c__DisplayClass80_0.<ProcessNode>b__0()
at Telerik.Windows.Documents.FormatProviders.Html.Import.HtmlDocumentImporter.ProcessNode(INode node)
at Telerik.Windows.Documents.FormatProviders.Html.Import.HtmlDocumentImporter.ProcessGenericNode(INode node)
at Telerik.Windows.Documents.FormatProviders.Html.Import.HtmlDocumentImporter.<>c__DisplayClass80_0.<ProcessNode>b__0()
at Telerik.Windows.Documents.FormatProviders.Html.Import.HtmlDocumentImporter.ProcessNode(INode node)
at Telerik.Windows.Documents.FormatProviders.Html.Import.HtmlDocumentImporter.ProcessGenericNode(INode node)
at Telerik.Windows.Documents.FormatProviders.Html.Import.HtmlDocumentImporter.<>c__DisplayClass80_0.<ProcessNode>b__0()
at Telerik.Windows.Documents.FormatProviders.Html.Import.HtmlDocumentImporter.ProcessNode(INode node)
at Telerik.Windows.Documents.FormatProviders.Html.Import.HtmlDocumentImporter.ProcessGenericNode(INode node)
at Telerik.Windows.Documents.FormatProviders.Html.Import.HtmlDocumentImporter.<>c__DisplayClass80_0.<ProcessNode>b__0()
at Telerik.Windows.Documents.FormatProviders.Html.Import.HtmlDocumentImporter.ProcessHyperlinkNode(INode node, Action innerAction)
at Telerik.Windows.Documents.FormatProviders.Html.Import.HtmlDocumentImporter.ProcessAnnotationsForNode(INode node, Action innerAction)
at Telerik.Windows.Documents.FormatProviders.Html.Import.HtmlDocumentImporter.ProcessNode(INode node)
at Telerik.Windows.Documents.FormatProviders.Html.Import.HtmlDocumentImporter.ProcessGenericNode(INode node)
at Telerik.Windows.Documents.FormatProviders.Html.Import.HtmlDocumentImporter.<>c__DisplayClass80_0.<ProcessNode>b__0()
at Telerik.Windows.Documents.FormatProviders.Html.Import.HtmlDocumentImporter.ProcessNode(INode node)
at Telerik.Windows.Documents.FormatProviders.Html.Import.HtmlDocumentImporter.Import(Stream input)
To work this around, avoid using the "var" function in the CSS.