Warning: ltrim() expects parameter 1 to be string

I’ve got a site with a Walks Pod implemented which contains the fields associated with a walk. It refers to 3 other Pods: an Area (which the walk is in), a Grade of Difficulty and a Start Place. When the administrator edits a walk each of the 3 fields relating to these other Pods has a warning just above it:

Warning: ltrim() expects parameter 1 to be string, object given in /var/sites/w/[websitename].org/public_html/wordpress/wp-includes/formatting.php on line 4313

Each of these is a drop-down list, and on one of the lists some of the entries (those which are children of another parent record) are shown with non-breaking space sequences, eg

   Tenerife

In the other ones, the values are just normal.

Although this warning appears, the administrator assures me that the walks appear ok on the site.

The site is on wp 5.4.1 and Pods 2.7.20.1 with Weaver Xtreme theme 4.0.5.

Any ideas how to get rid of the warning?