WebApplicationProxy.cs GetHttpApplicationType() exception

Just by doing this in my xUnit test:
[Fact]
public void TestMethod1()
{
WebApplicationProxy.Create(typeof(Studio.StudioLogin));
}

I am getting this exception in WebApplicationProxy.cs GetHttpApplicationType() method.

'_Instance._compiler.GetCompiledType("/Default.aspx")' threw an exception of type 'System.Web.HttpException'

A bit more detail - System.InvalidOperationException : The WebApplicationProxy has not been created and initialized properly

So it seems like I'm missing some crucial step here. Is there something wrong with the web application? It is a rather old and complicated legacy application with some non-standard techniques being used.
@csharpfritz

Start with WebApplicationProxy.cs, specifically GetHttpApplicationType(), and reproduce the exception using the xUnit test shown in the issue with WebApplicationProxy.Create(typeof(Studio.StudioLogin)). Trace the initialization state around _Instance._compiler and the /Default.aspx lookup. Done means identifying why the proxy is not initialized properly and documenting a verified fix or required setup.

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