fix(module:editor): fix destroying error when editor is not initialized

[wzhudev]

PR Checklist

Please check if your PR fulfills the following requirements:

• The commit message follows our guidelines: https://github.com/NG-ZORRO/ng-zorro-antd/blob/master/CONTRIBUTING.md#commit
• Tests for the changes have been added (for bug fixes / features)
• Docs have been added / updated (for bug fixes / features)

PR Type

What kind of change does this PR introduce?

[x] Bugfix
[ ] Feature
[ ] Code style update (formatting, local variables)
[ ] Refactoring (no functional changes, no api changes)
[ ] Build related changes
[ ] CI related changes
[ ] Documentation content changes
[ ] Application (the showcase website) / infrastructure changes
[ ] Other... Please describe:

What is the current behavior?

Issue Number: N/A

Open code editor's page and quickly switch to another component's page before monaco editor is rendered.

What is the new behavior?

When the editor is destroying, it checks if editorInstance is truthy before calling dispose method on it.

Does this PR introduce a breaking change?

[ ] Yes
[x] No

Other information

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[vthinkxie]

LGTM