NRE in SubStepsDispatcher

[rolfbjarne]

Test case: linkercustomsteptestcase-eb50a89.zip

Run the test.sh script to repro:

./test.sh
+ rm -Rf CustomStep/bin Executable/bin CustomStep/obj Executable/obj
+ dotnet build CustomStep/dotnet-linker.csproj
Microsoft (R) Build Engine version 16.8.0-preview-20425-03+384d02a5f for .NET
Copyright (C) Microsoft Corporation. All rights reserved.

  Determining projects to restore...
  Restored /Users/rolf/test/linkercustomsteptestcase/CustomStep/dotnet-linker.csproj (in 96 ms).
  You are using a preview version of .NET. See: https://aka.ms/dotnet-core-preview
  dotnet-linker -> /Users/rolf/test/linkercustomsteptestcase/CustomStep/bin/Debug/net5.0/dotnet-linker.dll

Build succeeded.
    0 Warning(s)
    0 Error(s)

Time Elapsed 00:00:01.11
+ dotnet publish -r osx-x64 --self-contained true -p:MonoAOT=true -p:VM=mono -p:PublishTrimmed=true -p:TrimMode=link Executable/Executable.csproj /bl:x.binlog
Microsoft (R) Build Engine version 16.8.0-preview-20425-03+384d02a5f for .NET
Copyright (C) Microsoft Corporation. All rights reserved.

/usr/local/share/dotnet/sdk/5.0.100-rc.1.20426.3/MSBuild.dll -distributedlogger:Microsoft.DotNet.Tools.MSBuild.MSBuildLogger,/usr/local/share/dotnet/sdk/5.0.100-rc.1.20426.3/dotnet.dll*Microsoft.DotNet.Tools.MSBuild.MSBuildForwardingLogger,/usr/local/share/dotnet/sdk/5.0.100-rc.1.20426.3/dotnet.dll -maxcpucount -property:RuntimeIdentifier=osx-x64 -property:SelfContained=true -p:MonoAOT=true -p:VM=mono -p:PublishTrimmed=true -p:TrimMode=link -restore -target:Publish -verbosity:m /bl:x.binlog Executable/Executable.csproj
  Determining projects to restore...
  Restored /Users/rolf/test/linkercustomsteptestcase/Executable/Executable.csproj (in 87 ms).
  You are using a preview version of .NET. See: https://aka.ms/dotnet-core-preview
  Executable -> /Users/rolf/test/linkercustomsteptestcase/Executable/bin/Debug/net5.0/osx-x64/Executable.dll
ILLink : error IL1012: IL Linker has encountered an unexpected error. Please report the issue at https://github.com/mono/linker/issues [/Users/rolf/test/linkercustomsteptestcase/Executable/Executable.csproj]
  Fatal error in IL Linker
  Unhandled exception. System.NullReferenceException: Object reference not set to an instance of an object.
     at Mono.Linker.Steps.SubStepsDispatcher.DispatchType(TypeDefinition type)
     at Mono.Linker.Steps.SubStepsDispatcher.BrowseTypes(Collection`1 types)
     at Mono.Linker.Steps.SubStepsDispatcher.BrowseAssemblies(IEnumerable`1 assemblies)
     at Mono.Linker.Steps.SubStepsDispatcher.Mono.Linker.Steps.IStep.Process(LinkContext context)
     at Mono.Linker.Pipeline.ProcessStep(LinkContext context, IStep step)
     at Mono.Linker.Pipeline.Process(LinkContext context)
     at Mono.Linker.Driver.Run(ILogger customLogger)
     at Mono.Linker.Driver.Main(String[] args)
  Optimizing assemblies for size, which may change the behavior of the app. Be sure to test after publishing. See: https://aka.ms/dotnet-illink
/usr/local/share/dotnet/sdk/5.0.100-rc.1.20426.3/Sdks/Microsoft.NET.Sdk/targets/Microsoft.NET.ILLink.targets(41,5): error NETSDK1144: Optimizing assemblies for size failed. Optimization can be disabled by setting the PublishTrimmed property to false. [/Users/rolf/test/linkercustomsteptestcase/Executable/Executable.csproj]

$ dotnet --version
5.0.100-rc.1.20426.3

This seems to be when there's one substep that includes types, another that doesn't and they're active for different assemblies.