PR Summary

This PR fixes a bug where a null reference exception is thrown during Start-Job, if default Windows PowerShell default shell Id is not in the registry.

PR Context

Some Azure images have run into this problem where Start-Job fails even when running in pwsh child process rather than Windows PowerShell child. Fix is to guard against the missing registry entry.

PR Checklist

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• This PR is ready to merge and is not Work in Progress.
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• Breaking changes
  • None
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  • Experimental feature(s) needed
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• User-facing changes
  • Not Applicable
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  • Documentation needed
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• Testing - New and feature
  • N/A or can only be tested interactively
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• Tooling
  • I have considered the user experience from a tooling perspective and don't believe tooling will be impacted.
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  • I have considered the user experience from a tooling perspective and enumerated concerns in the summary. This may include:
    • Impact on PowerShell Editor Services which is used in the PowerShell extension for VSCode (which runs in a different PS Host).
    • Impact on Completions (both in the console and in editors) - one of PowerShell's most powerful features.
    • Impact on PSScriptAnalyzer (which provides linting & formatting in the editor extensions).
    • Impact on EditorSyntax (which provides syntax highlighting with in VSCode, GitHub, and many other editors).