By default, the ThreadHelper.JoinableTaskFactory and AsyncPackage.JoinableTaskFactory properties only works for code running in the VS process.
Unit testing your code that relies on these properties may throw exceptions.
Important: Your product code should never instantiate its own JoinableTaskContext.
Always use the one from ThreadHelper.JoinableTaskContext.
To get ThreadHelper (and AsyncPackage) to work within a unit test process, you may use the VS SDK Test Framework.
This framework includes instructions both for MSTest and Xunit to enable your tests to run code that includes usage of ThreadHelper, the global IServiceProvider, and other APIs that typically only work in the VS process.