PHP8 warning in theme_config->editor_css_files()

Problem F: PHP8 warning in theme_config->editor_css_files(). It happens 12 times.

160656:[Tue Nov 15 09:02:05.331079 2022] [php:notice] [pid 4853] [client 172.28.0.6:33522] Default exception handler: Exception - Warning: foreach() argument must be of type array|object, null given in [dirroot]/lib/outputlib.php on line 968 Debug:
Error code: generalexceptionmessage

• line 157 of /lib/behat/lib.php: Exception thrown
• line 968 of /lib/outputlib.php: call to behat_error_handler()
• line 1331 of /lib/outputlib.php: call to theme_config->editor_css_files()
• line 129 of /theme/styles.php: call to theme_config->get_css_content_editor()
  , referer: http://webserver131d771ed741fc3b/behatrun3/course/modedit.php?add=data&type=&course=100000&section=1&return=0&sr=0