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

---

What is the purpose of this pull request?

• Bug fix
• New Feature
• Documentation update
• Other

Before submitting the PR, please make sure you do the following

• Read the Contributing Guidelines.
• Read the Pull Request Guidelines and follow the Commit Convention.
• Check that there isn't already a PR that solves the problem the same way to avoid creating a duplicate.
• Provide a description in this PR that addresses what the PR is solving, or reference the issue that it solves (e.g. fixes #123).