Use null.

src/Workspaces/Core/Portable/EmbeddedLanguages/RegularExpressions/LanguageServices/RegexEmbeddedBraceMatcher.cs

@@ -32,7 +32,7 @@ public RegexEmbeddedBraceMatcher(RegexEmbeddedLanguage language)
                 RegularExpressionsOptions.HighlightRelatedRegexComponentsUnderCursor, document.Project.Language);
             if (!option)
             {
-                return default;
+                return null;
             }
 
             var root = await document.GetSyntaxRootAsync(cancellationToken).ConfigureAwait(false);