Building main with Castle.Core 5.2.1 gives build errors (Possible null reference)

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See #870

Error: /home/runner/work/NSubstitute/NSubstitute/src/NSubstitute/Proxies/CastleDynamicProxy/ProxyIdInterceptor.cs(28,29): error CS8602: Dereference of a possibly null reference. [/home/runner/work/NSubstitute/NSubstitute/src/NSubstitute/NSubstitute.csproj::TargetFramework=net8.0]
Error: /home/runner/work/NSubstitute/NSubstitute/src/NSubstitute/Proxies/CastleDynamicProxy/CastleInvocationMapper.cs(12,13): error CS8602: Dereference of a possibly null reference. [/home/runner/work/NSubstitute/NSubstitute/src/NSubstitute/NSubstitute.csproj::TargetFramework=net8.0]
Error: /home/runner/work/NSubstitute/NSubstitute/src/NSubstitute/Proxies/CastleDynamicProxy/CastleInvocationMapper.cs(27,72): error CS8603: Possible null reference return. [/home/runner/work/NSubstitute/NSubstitute/src/NSubstitute/NSubstitute.csproj::TargetFramework=net8.0]
Error: /home/runner/work/NSubstitute/NSubstitute/src/NSubstitute/Proxies/CastleDynamicProxy/CastleDynamicProxyFactory.cs(128,26): error CS8604: Possible null reference argument for parameter 'target' in 'object ProxyGenerator.CreateInterfaceProxyWithTarget(Type interfaceToProxy, Type[]? additionalInterfacesToProxy, object target, ProxyGenerationOptions options, params IInterceptor[] interceptors)'. [/home/runner/work/NSubstitute/NSubstitute/src/NSubstitute/NSubstitute.csproj::TargetFramework=net8.0]

See logs: https://github.com/nsubstitute/NSubstitute/actions/runs/13752672225/job/38455602274?pr=870

PS: There is a comment about
Castle.Core 5.2 and NSubstitute, but that seems about something else. castleproject/Core#684 (comment)

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I haven't tested NSubstitute locally with Castle.Core 5.2.1 yet. Maybe it isn't a runtime error with NSubstitute 5.3.0