[Client] Nil-dereference of client API call fixed

[hyunsooda]

Proposed changes

This PR aims to address potential nil dereference issues and prevent crashes caused by invalid input. The changes made in this PR ensure that if a value is found to be nil, an appropriate error is returned instead of attempting to proceed with the nil value (crash).

Types of changes

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• Bugfix
• New feature or enhancement
• Others

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• Lint and unit tests pass locally with my changes ($ make test)
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Related issues

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Further comments

If this is a relatively large or complex change, kick off the discussion by explaining why you chose the solution you did and what alternatives you considered, etc...

[aidan-kwon]

@kjeom PTAL