When configuring an instance of Flickr Public repository, when the 'usewatermarks' checkbox is cleared (the default) this error is generated:

Undefined property: stdClass::$usewatermarks in moodle\admin\repositoryinstance.php on line 71

If the checkbox is checked, there are no errors. However, the 'usewatermarks' setting in the database remains set to 1 and can't be unset using the moodle admin form.

Originally there was also another error when the checkbox was unchecked:

Undefined index: usewatermarks in moodle\repository\lib.php on line 1247

This issue is resolved by adding a default value for the checkbox in function 'instance_config_form' in moodle\repository\flickr_public\lib.php

insert at line 459: $mform->setDefault('usewatermarks', 0);