anchor.diff:      let doc = new FSharpParsedDocument(fileName)
       doc.Flags <- doc.Flags ||| ParsedDocumentFlags.NonSerializable
       let errors = 
  +      match fileName with
  +      | null -> []
  +      | _ ->
           match prevErrors.TryGetValue(fileName) with 
           | true,err -> 
               prevContent.Remove(fileName) |> ignore
  
positive.code:     // Trigger a parse/typecheck in the background. After the parse/typecheck is completed, request another parse to report the errors.
      //
      // Skip this is this call is a result of updating errors and the content still matches.
      if not (prevContent.ContainsKey(fileName) && prevContent.[fileName] = fileContent ) && Common.supportedExtension(IO.Path.GetExtension(fileName)) then 
        // Trigger parsing in the language service 
        let filePathOpt = 
            // TriggerParse will work only for full paths
  
negative.code:         if config <> null then 
            LanguageService.Service.TriggerParse(filePath, fileContent, proj, config, full=false, afterCompleteTypeCheckCallback=(fun (fileName,errors) ->
  
                      let file = fileName.FullPath.ToString()
                      if file <> null then
                        prevErrors.[file] <- errors
                        prevContent.[file] <- fileContent
                        // Scheule another parse to actually update the errors 
  