fix: editor instance is undefined

[HellowVirgil]

Description

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Type of Change

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• Bug fix
• New feature
• Documentation update
• Code refactoring
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Related Issues

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Testing

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Checklist

• My code follows the project’s coding style.
• I have updated the documentation as necessary.
• I have added tests to cover my changes.
• All new and existing tests pass.
• My changes require a change to the documentation.

Additional Notes

如果组件被挂载（mount）后立即被卸载（unmount），或者在 getOrCreateEditorState 异步操作完成之前触发了 onViewUnmount ，此时 this.editor 尚未被赋值，仍然是 undefined ，导致报错。

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