TypeError: plainDirectives.flatMap is not a function

[icnocop]

When I try to generate an AutoRest C# client from AutoRestClient.json, I get the following error:

(node:18000) ExperimentalWarning: The fs.promises API is experimental
  TypeError: plainDirectives.flatMap is not a function


   at ChristianHelle.DeveloperTools.CodeGenerators.ApiClient.Core.Generators.ProcessLauncher.StartInternal(String command, String arguments, Action`1 onOutputData, Action`1 onErrorData, String workingDirectory)
   at ChristianHelle.DeveloperTools.CodeGenerators.ApiClient.Core.Generators.ProcessLauncher.Start(String command, String arguments, Action`1 onOutputData, Action`1 onErrorData, String workingDirectory)
   at ChristianHelle.DeveloperTools.CodeGenerators.ApiClient.Core.Generators.ProcessLauncher.Start(String command, String arguments, String workingDirectory)
   at ChristianHelle.DeveloperTools.CodeGenerators.ApiClient.Core.Generators.AutoRest.AutoRestCSharpCodeGenerator.OnGenerateCode(IProgressReporter pGenerateProgress)
   at ChristianHelle.DeveloperTools.CodeGenerators.ApiClient.Core.Generators.AutoRest.AutoRestCSharpCodeGenerator.GenerateCode(IProgressReporter pGenerateProgress)
   at ChristianHelle.DeveloperTools.CodeGenerators.ApiClient.CustomTool.SingleFileCodeGenerator.Generate(String wszInputFilePath, String bstrInputFileContents, String wszDefaultNamespace, IntPtr[] rgbOutputFileContents, UInt32& pcbOutput, IVsGeneratorProgress pGenerateProgress)

Support Key: 14a9a75

Steps to reproduce

1. Visual Studio 2019 Enterprise Edition
2. Create a new C# library project which targets .net standard 2.0
3. Right-click on the project and select Add > New REST API Client > Generate with AutoRest
4. Enter URL of swagger.json
5. Enter Filename AutoRestClient
6. Click OK
   I expected the client to get generated here, but it didn't.
   The Output window indicated the "Microsoft.Azure.AutoRest.CSharp" nuget package installation failed.
   I had to add this to my csproj to be able to build:
   <LangVersion>8.0</LangVersion>
7. Right-click on AutoRestClient.json and select "Run Custom Tool"
   Error appeared in the Output window.

Expected behavior
I expected the REST API client class to be generated.

Screenshots

Visual Studio Extension:

• Visual Studio 2019
• REST API Client Code Generator 1.2.7037

Additional context
Thank you!

Edit FYI, as a work-around, I'm now using https://devblogs.microsoft.com/aspnet/generating-http-api-clients-using-visual-studio-connected-services/ instead.