CsWinRT fails during code generation with NullReferenceException

[ppoirier1]

Describe the bug
Build fails with the following during code generation:
CSC : warning CS8785: Generator 'SourceGenerator' failed to generate source. It will not contribute to the output and compilation errors may occur as a result. Exception was of type 'NullReferenceException' with message 'Object reference not set to an instance of an object.'.

Version Info
CsWinRT version 2.1.1 and higher
Windows SDK 22621

Additional context
Broke at 2.1.1. Version 2.0.8 and earlier versions worked fine. Tried up to the current version 2.1.6
Log.txt:
...
output: C:\Users\user\AppData\Local\Temp\CsWinRT\q22y5fj2.4ha
time: 65ms

Adding C:\Users\user\AppData\Local\Temp\CsWinRT\q22y5fj2.4ha\AuthoringMetadataTypeMappingHelper.cs
Adding C:\Users\user\AppData\Local\Temp\CsWinRT\q22y5fj2.4ha\InitalizeProjection.cs
Adding C:\Users\user\AppData\Local\Temp\CsWinRT\q22y5fj2.4ha\Tasks.cs
Adding C:\Users\user\AppData\Local\Temp\CsWinRT\q22y5fj2.4ha\WinRTEventHelpers.cs
Adding C:\Users\user\AppData\Local\Temp\CsWinRT\q22y5fj2.4ha\WinRT_Module.cs
System.NullReferenceException: Object reference not set to an instance of an object.
at Generator.WinRTTypeWriter.<>c.b__80_1(VtableAttribute vtableAttribute, Int32 _)
at System.Linq.Enumerable.d__192.MoveNext() at System.Linq.Enumerable.<UnionIterator>d__671.MoveNext()
at System.Linq.Enumerable.d__641.MoveNext() at System.Linq.Buffer1..ctor(IEnumerable1 source) at System.Linq.Enumerable.ToArray[TSource](IEnumerable1 source)
at System.Collections.Immutable.ImmutableArray.CreateRange[T](IEnumerable`1 items)
at Generator.WinRTTypeWriter.GenerateWinRTExposedClassAttributes(GeneratorExecutionContext context)
at Generator.ComponentGenerator.Generate()