NullReferenceException crashing docfx build on Azure DevOps

[CodeBlanch]

Operating System: Windows

DocFX Version Used: 2.56.6

Template used: default

Expected Behavior:

No errors.

Actual Behavior:

A bunch of LoaderExceptions and then a NullReferenceException.

[21-02-26 05:24:16.951]Warning:MetadataCommand.ExtractMetadataError opening project C:/vsts-agent-win-x64-2.172.2/_work/19/s/Project.csproj: Unable to load one or more of the requested types. Retrieve the LoaderExceptions property for more information.. Ignored.

[21-02-26 05:24:16.951]Error:Error extracting metadata for C:/vsts-agent-win-x64-2.172.2/_work/19/s/Project.csproj: System.NullReferenceException: Object reference not set to an instance of an object.
at Microsoft.DocAsCode.Metadata.ManagedReference.ExtractMetadataWorker.d__13.MoveNext()
--- End of stack trace from previous location where exception was thrown ---
at System.Runtime.ExceptionServices.ExceptionDispatchInfo.Throw()
at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task)
at Microsoft.DocAsCode.Metadata.ManagedReference.ExtractMetadataWorker.d__11.MoveNext()

Analysis:

I did a bunch of digging on this and tried what was suggested on #4869 & #4782, but I think this is something different. With verbose logging turned on:

[21-02-26 05:24:15.831]Info:[MetadataCommand.ExtractMetadata]Using msbuild C:\Program Files (x86)\Microsoft Visual Studio\2019\ TestAgent \MSBuild\Current\Bin as inner compiler.

Notice the highlight there, the test version of VS is being selected.

It looks like this was a bug fixed in MSBuildLocator: microsoft/MSBuildLocator#73

A possible fix is to update the dependency docfx has to >=1.3.2?

[filzrev]

I thought this issue can be closed by following reasons

• docfx v2.59.4 is using bug fixed version (MSBuildLocator 1.41)
• Recent version of docfx has no VS instance dependencies