To reproduce in Moodle 2.6 latest weekly build:

• Fresh Moodle Install
• with course creation tool create two "M" sized courses in the "Miscellaneous" category
• from the 'Course and category management' page delete 1 course
• going back to the course 'Course and category management' with Course and category visible, the following errors appears:

PHP catchable fatal error
Debug info: Argument 1 passed to course_in_list::__construct() must be an instance of stdClass, null given, called in [dirroot]/lib/coursecatlib.php on line 1433 and defined
Error code: codingerror

Stack trace:

line 393 of /lib/setuplib.php: coding_exception thrown
line 2644 of /lib/coursecatlib.php: call to default_error_handler()
line 1433 of /lib/coursecatlib.php: call to course_in_list->__construct()
line 517 of /course/classes/management_renderer.php: call to coursecat->get_courses()
line 497 of /course/management.php: call to core_course_management_renderer->course_listing()

The error is most probably related to caching as we noticed after course deletion of one of two courses the category 'Miscellaneous' was still showing 2 courses inside itself.

Please note the error has been reproduced with default Moodle cache store (i.e. no memcache or other store used).