[DateTimeModel] Test Microsoft.Recognizers.Text.DateTime.Tests. TestDateTime_German DateTimeModel occasionally fail

[MichaelMWW]

Describe the bug
Test Microsoft.Recognizers.Text.DateTime.Tests. TestDateTime_German DateTimeModel occasionally fail

To Reproduce
Steps to reproduce the behavior:

1. Open the Microsoft.Recognizers.Text.sln solution under .NET folder, build the solution.
2. Open the Test Explorer, find the test Microsoft.Recognizers.Text.DateTime.Tests. TestDateTime_German DateTimeModel, right click it and click Run Until Failure, the issue can be repro, more details please refer below image.
   

Expected behavior
The test passed

Sample input/output
Test failed occasionally, it is an issue not always repro.

Failed build:
https://ci.appveyor.com/project/tellarin/recognizers-text/builds/50375089

Error message:
Failed Microsoft.Recognizers.Text.DateTime.Tests.TestDateTime_German DateTimeModel [13 ms]
Error Message:
Test method Microsoft.Recognizers.Text.DateTime.Tests.TestDateTime_German.DateTimeModel threw exception:
System.ApplicationException: Input: "Tag der Arbeit, ersten Weihnachtsfeiertag, zweiten Weihnachtstag" ---> System.NullReferenceException: Object reference not set to an instance of an object.
Stack Trace:
at Microsoft.Recognizers.Text.DataDrivenTests.TestBase.TestDateTime(TestModel testSpec) in C:\projects\recognizers-text.NET\Microsoft.Recognizers.Text.DataDrivenTests\TestBase.cs:line 87
--- End of inner exception stack trace ---
at Microsoft.Recognizers.Text.DataDrivenTests.TestBase.TestDateTime(TestModel testSpec) in C:\projects\recognizers-text.NET\Microsoft.Recognizers.Text.DataDrivenTests\TestBase.cs:line 138
at Microsoft.Recognizers.Text.DateTime.Tests.TestDateTime_German.DateTimeModel(TestModel testSpec) in C:\projects\recognizers-text.NET\Microsoft.Recognizers.Text.DataDrivenTests\DateTime\TestDateTime_German.cs:line 206

Platform (please complete the following information):

• Platform: .NET
• Environment: nuget package
• Version of package v1.0.8.7

Additional context
Add any other context about the problem here.