Catchable fatal error

Hello,

I get this error :

Catchable fatal error: Object of class WP_Error could not be converted to string in /home/alysse/webapps/remorques/wp-content/themes/generatepress_child/content-sel-page.php on line 22

when I try to edit the page (custom page) in YellowPencil plugin.

Line 21-24 :

<header class="entry-header">
<?php $montitre = "<h1>".get_the_category_by_ID($catp)." / ".get_the_category_by_ID($cats)."</h1>"; ?>
<?php echo $montitre ?>
</header>

No error when I see the page.

I ask to the YellowPencil support :

Answer :
Hi, this error is related to your child theme and not to plugin.
Try contacting your theme developers about this issue.