Session mutation bugs with debugging on

[brendanheywood]

I'm getting these bugs fairly often, but not 100% of the time when editing templates. I have show debugging on and

Warning: Undefined array key 0 in /var/www/master/mod/cms/classes/local/renderer.php on line 87
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Error output, so disabling automatic redirect.

Script /mod/cms/managetypes.php?action=edit&id=1 mutated the session after it was closed: $SESSION->notifications
line 791 of /lib/classes/session/manager.php: call to debugging()
line ? of unknownfile: call to core\session\manager::check_mutated_closed_session()
line 159 of /lib/classes/shutdown_manager.php: call to call_user_func_array()
line ? of unknownfile: call to core_shutdown_manager::shutdown_handler()

[marcusboon]

Probably happens if customfields and/or userlist is checked but no fields are defined