Issues with swagger after migrating to .NET Core 3.0

There is a discussion on the aspnet/AspNetCore github repo discussing this: Restore MvcJsonOptions to ASP.NET Core 3.0 as a type forward?

It suggests the issue was resolved.

I'd suggest ensuring you have the latest Swashbuckle.AspnetCore package installed:

In the offical core docs, it states:

Install-Package Swashbuckle.AspNetCore -Version 5.0.0-rc2

So I'd suggest you need to install a pre-release version, at the time of writing only 4.X is stable and 5.0.0-rc3 is the latest.