Problem with video post format: “Undefined variable: content_parts”

Hi! I’m having problems with the video post format. There’s an old topic about this but the workaround there doesn’t seem to work anymore.
I’m using Fukasawa 1.22 and WordPress 5.1.1.

The latest ReadMe info says:

== Use the video post format
1. Go to Admin > Posts > Add New.
2. Select the "Video" post format in the Post Attributes box.
3. A meta box with the title "Video URL" should appear in the top of the right sidebar.
4. Paste the URL to the video you wish to display in the box, and publish/update the post.
5. The video you linked to should now be displayed in the media section above the post.

Number 3 doesn’t happen for me.

If I publish a post in the video format, and I embed a YouTube video in it, when I go see the post there’s only the title, and no content whatsoever.

Query monitor shows that singular.php at line 77 gives this error: Undefined variable: content_parts.

Something to do with:

<?php 
if ( $post_format == 'video' ) { 
$content = $content_parts['extended'];
$content = apply_filters( 'the_content', $content );
echo $content;
} else {
the_content();
}
?>

Can anyone help?