NSWag stops service from starting in .NET 9

[arisewanggithub]

Describe the bug
With .NET 9, NSwag throws NullReferenceException in ShouldSerializeXXX methods in data model in service start. I opened a bug with asp.net core. And they said it should be a NSwag issue.
Here is the asp.net issue link: dotnet/aspnetcore#59402

A clear and concise description of what the bug is.
ShouldSerializeXXX methods in data model throws NullReferenceException on service startup in .NET 9

Version used
14.2.0

With which version did you encounter this behavior.

To Reproduce
checkout the sample code at: https://github.com/arisewanggithub/dotnetissue59402

Ideally code samples or event unit test.

Expected behavior
The same code (which works in .NET 8) should continue to work in .NET 9.

A clear and concise description of what you expected to happen.

Additional context

Add any other context about the problem here.