fix stage simulator to create tradable material in case of circle · planetarium/lib9c@bd404cf

The following actions use a deprecated Node.js version and will be forced to run on node20: actions/setup-dotnet@v3. For more info: https://github.blog/changelog/2024-03-07-github-actions-all-actions-will-run-on-node20-instead-of-node16-by-default/

build: Lib9c.Policy/NCStagePolicy.cs#L42

Possible null reference return.

build: Lib9c.Policy/Policy/DebugPolicy.cs#L22

Possible null reference return.

build: Lib9c.Policy/Policy/DebugPolicy.cs#L28

Possible null reference return.

build: Lib9c.Policy/Policy/MinBlockProtocolVersionPolicy.cs#L25

Cannot convert null literal to non-nullable reference type.

build: Lib9c.Policy/Policy/MinBlockProtocolVersionPolicy.cs#L33

Cannot convert null literal to non-nullable reference type.

build: Lib9c.Policy/Policy/SpannedSubPolicy.cs#L45

Cannot convert null literal to non-nullable reference type.

build: Lib9c.Policy/Policy/VariableSubPolicy.cs#L138

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

build: Lib9c.Policy/Policy/BlockPolicySource.Utils.cs#L108

Possible null reference return.

build: Lib9c.Policy/Policy/BlockPolicySource.Utils.cs#L138

Possible null reference return.

build: Lib9c.Policy/Policy/BlockPolicySource.Utils.cs#L165

Possible null reference return.

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