Fix broken tests after merge · planetarium/lib9c@b3a63f2

Node.js 16 actions are deprecated. Please update the following actions to use Node.js 20: actions/setup-dotnet@v3. For more information see: https://github.blog/changelog/2023-09-22-github-actions-transitioning-from-node-16-to-node-20/.

Possible null reference return.

Possible null reference return.

Possible null reference return.

Cannot convert null literal to non-nullable reference type.

Cannot convert null literal to non-nullable reference type.

Cannot convert null literal to non-nullable reference type.

Possible null reference return.

Converting null literal or possible null value to non-nullable type.

Possible null reference return.

Possible null reference return.

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