PR Summary

Fix #11227, NullReferenceException in CompletionCompleters public methods:

[CompletionCompleters]::CompleteType(string typeName)
[CompletionCompleters]::CompleteVariable(string variableName)

PR Context

PR Checklist

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• Breaking changes
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• Testing - New and feature
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• Tooling
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    • 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).
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