fix(iframe): Add optional chaining on unsafe-type element

[Azurewarth0920]

Description

Fix: #2078

Check the existence of the root element as the root element may be empty. (Eg. described in #2078, using custom render that doesn't generate DOM may cause the empty root)

Additional context

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What is the purpose of this pull request?

• [x] Bug fix
• [ ] New Feature
• [ ] Documentation update
• [ ] Other

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• [x] Provide a description in this PR that addresses what the PR is solving, or reference the issue that it solves (e.g. fixes #123).