Fatal Error

WordPress sent me a warning about a fatal error related to PhotoGallery. They sent details an dI checked error logs of the site where I found similar messages. This is what WordPress included with the warning:

Een fout van het type E_ERROR werd veroorzaakt op regelnummer 557 van het bestand /home/deb141677/domains/kunstmarktwijchen.nl/public_html/wp-content/plugins/photo-gallery/admin/models/Galleries.php. Foutmelding: Uncaught Error: Attempt to assign property "author" on null in /home/deb141677/domains/kunstmarktwijchen.nl/public_html/wp-content/plugins/photo-gallery/admin/models/Galleries.php:557
Stack trace:
#0 /home/deb141677/domains/kunstmarktwijchen.nl/public_html/wp-content/plugins/photo-gallery/admin/controllers/Galleries.php(327): GalleriesModel_bwg->get_row_data()
#1 /home/deb141677/domains/kunstmarktwijchen.nl/public_html/wp-content/plugins/photo-gallery/admin/controllers/Galleries.php(82): GalleriesController_bwg->edit()
#2 /home/deb141677/domains/kunstmarktwijchen.nl/public_html/wp-content/plugins/photo-gallery/photo-gallery.php(593): GalleriesController_bwg->execute()
#3 /home/deb141677/domains/kunstmarktwijchen.nl/public_html/wp-includes/class-wp-hook.php(324): BWG->admin_pages()
#4 /home/deb141677/domains/kunstmarktwijchen.nl/public_html/wp-includes/class-wp-hook.php(348): WP_Hook->apply_filters()
#5 /home/deb141677/domains/kunstmarktwijchen.nl/public_html/wp-includes/plugin.php(517): WP_Hook->do_action()
#6 /home/deb141677/domains/kunstmarktwijchen.nl/public_html/wp-admin/admin.php(259): do_action()
#7 {main}
?thrown

I cannot really understand this but I interpret it as relating to the admin-side of the plugin.

If this is correct, could a problem have been caused by a cancelled action to install the 10Web Booster plugin. I started doing this but after a few steps realized this would do something similar as other plugins I already use. So I stopped the process halfway.

I discovered some folders in de wp-content directory that seem to point in this direction. (see image). If some things were installed but the process was not completed, I can imagine this causes conflicts at the backend.

If this is indeed the case, can you please tell me how solve the issue?

Or, if the cause lies elsewhere, what can it be? I have already inactivated most plugins and checked php.

Thank you for your help.

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