E_WARNING: Creating default object from empty value

Stack trace
in unknown called at /var/www/html/mod/assign/version.php (27)
in include called at /var/www/html/webservice/externallib.php (142)
in core_webservice_external::get_site_info called at ?
in call_user_func_array called at /var/www/html/webservice/lib.php (1780)
in webservice_base_server::execute called at /var/www/html/webservice/lib.php (1644)
…webservice_base_server::run called at /var/www/html/webservice/rest/
server.php (44)

Reproducible steps

Pre-requisites

1. A user configured to use an external web service that includes core_webservice_get_site_info, for example Moodle Mobile sevices
2. You have a web service token for the user.
3. Moodle and PHP are set to log Warnings

Steps

1. Load the webservice via <your moodle url>/webservice/rest/server.php?moodlewsrestformat=xml&wstoken=<web service token>&wsfunction=core_webservice_get_site_info
2. Examine your PHP error log
3. You should see an warning about Creating default object from empty value