Microsoft.AspNetCore.Mvc.Analyzers.TopLevelParameterNameAnalyzer threw an exception of type 'System.NullReferenceException'

I'm updating a package to .NET 5.0 from .Net Core 2.1. I've updated all the assemblies to the latest versions but am getting the error below from the Analyzer package:

This error occurs due to the inclusion of some other packages that were previously updated to .NET 5.0. The offending packages all include the following in their project file:

  <ItemGroup>
    <FrameworkReference Include="Microsoft.AspNetCore.App" />
  </ItemGroup>

This gives rise to 2 versions of Microsoft.AspNetCore.Mvc.Analyzers appearing in my project dependencies and hence I get the error.

Anyway short of updating all the packages to remove the need for them to include Microsoft.AspNetCore.App (beyond the scope of the current development) has anyone any suggestions of how to solve this issue? I'm under the assumption that it is fine to specify this FrameworkReference include in a .Net 5.0 project when it has dependencies on core libraries.

It turns out that way down in the dependency hierarchy there were some references hanging around referencing deprecated Microsoft.AspNetCore packages even though the package itself had been switched to target .NET 5.

I found the offending packages by looking at the dependencies in the Solution Explorer in VS.

I removed these and replaced them with the single Framework reference as seen below:

And Uncle Bob's your uncle it works now.