fix: if activePath is null, use link value

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Name	Status	Preview	Comments	Updated (UTC)
dumi-theme-antd-style	✅ Ready (Inspect)	Visit Preview	💬 Add feedback	Apr 3, 2023 8:59am

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:tada: This PR is included in version 0.21.1-beta.1 :tada:

The release is available on:

• npm package (@beta dist-tag)
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