Operations that change non-concurrent collections must have exclusive access

[danipen]

https://forum.plasticscm.com/topic/23292-operations-that-change-non-concurrent-collections-must-have-exclusive-access/#comment-45776

This suddenly popped up when I alt-tabbed back to plastic. I'd changed a source file that I think was visible in the diff view.
I could continue working after forcibly killing plastic. What exactly has been corrupted and should I be concerned?

DEBUG ExceptionHandler - Stack trace:    at System.Collections.Generic.Dictionary`2.FindValue(TKey key)
   at TextMateSharp.Themes.Theme.Match(IList`1 scopeNames) in /_/src/TextMateSharp/Themes/Theme.cs:line 42
   at AvaloniaEdit.TextMate.TextMateColoringTransformer.ProcessTokens(Int32 lineNumber, List`1 tokens)
   at AvaloniaEdit.TextMate.TextMateColoringTransformer.TransformLine(DocumentLine line, ITextRunConstructionContext context)