Link button doesn't insert URL

If I’m using the WP Editor post editor and highlight some text, then click the ‘link’ button and fill in a URL, when I’m done, the link always looks like this: <a href="undefined" title="undefined">

With the built-in editor, the URL is filled in.

Any idea how to fix this?

https://ww.wp.xz.cn/plugins/wp-editor/