Error thrown when running tests with Xunit in parallel

[braingame26]

I sometimes get the below error when running my tests with xunit.

System.NullReferenceException: Object reference not set to an instance of an object.
at TestStack.BDDfy.Reporters.FileReportSummaryModel.<>c.<.ctor>b__2_0(Story s)
at System.Linq.Enumerable.d__162.MoveNext() at System.Collections.Generic.List1..ctor(IEnumerable1 collection) at System.Linq.Enumerable.ToList[TSource](IEnumerable1 source)
at TestStack.BDDfy.Reporters.FileReportSummaryModel..ctor(ReportModel reportModel)
at TestStack.BDDfy.Reporters.FileReportModel..ctor(ReportModel reportModel)
at TestStack.BDDfy.Reporters.Html.HtmlReportModel..ctor(IHtmlReportConfiguration configuration, ReportModel reportModel)
at TestStack.BDDfy.Reporters.Html.HtmlReporter.WriteOutHtmlReport(ReportModel reportModel)
at TestStack.BDDfy.Reporters.Html.HtmlReporter.Process(IEnumerable`1 stories)
at TestStack.BDDfy.Engine.InvokeBatchProcessors()
at TestStack.BDDfy.Engine.<>c.<.cctor>b__1_0(Object sender, EventArgs e)

I've included a sample project so that you can reproduce the error.
bddfy_error.zip

The xunit.console executable I'm using is in the XUnit folder.

Like I said, the error only happens sometimes. I've created a little script that runs the tests 100 times. For me, one of those times it throws the error. The script is run.ps1 at PCL.Tests/bin/Debug.