Guard against dead variables container in compact object/instance editors

[4ian]

Motivation

• Prevent crashes when accessing variables of objects or instances that may have become invalid by guarding against dead pointers returned by object.getVariables() or instance.getVariables().
• Ensure the variables list UI only renders when a valid variables container is available to avoid undefined behavior in the compact editors.

Description

• Import exceptionallyGuardAgainstDeadObject from ../../Utils/IsNullPtr and use it to sanitize object.getVariables() and instance.getVariables() into variablesContainer before use.
• Use variablesContainer instead of direct calls to getVariables() when rendering VariablesList and when handling the object variables TopLevelCollapsibleSection to conditionally render that section only when the container is valid.
• Keep the variables list ref and add behavior intact while preventing usage of a potentially dead object pointer.

Testing

• Ran the project's test suite with yarn test, and all tests passed.
• Ran the linter with yarn lint, and no linting errors were reported.
• Performed a development build with yarn build, which completed successfully.

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Codex Task