Fatal error with Gutenberg plugin

This theme has a conflict with the Gutenberg plugin that causes the fatal error below when trying to edit content with the Gutenberg editor.

Fatal error: Uncaught Error: [] operator not supported for strings in /home/user/Sites/site.test/site/web/app/plugins/gutenberg/lib/global-styles.php:226 Stack trace: #0 /home/user/Sites/site.test/site/web/wp/wp-includes/class-wp-hook.php(289): gutenberg_experimental_global_styles_settings() #1 /home/user/Sites/site.test/site/web/wp/wp-includes/plugin.php(212): WP_Hook->apply_filters() #2 /home/user/Sites/site.test/site/web/wp/wp-admin/edit-form-blocks.php(415): apply_filters() #3 /home/user/Sites/site.test/site/web/wp/wp-admin/post.php(187): require(‘/home/user/Site…’) #4 /home/user/.config/composer/vendor/cpriego/valet-linux/server.php(232): require(‘/home/user/Site…’) #5 {main} thrown in /home/user/Sites/site.test/site/web/app/plugins/gutenberg/lib/global-styles.php on line 226

This is caused by the harrison_block_editor_settings() function in harrison/inc/gutenberg.php line 133. Commenting out the filter hook on line 141 fixes the problem. Alternatively, it can be fixed by including add_theme_support( ‘editor-styles’ ) in a child theme.