Before this patch, the bigbluebuttonbn_print_overview_element() function, which is called from bigbluebuttonbn_print_overview() which is deprecated since Moodle 3.8 but which is still called on previous Moodle core versions and from old-style Moodle Dashboard blocks, didn't check if $bigbluebuttonbn->closingtime is defined in the particular BBB activity instance or not.

As a result, users saw this line for BBB activity instances with a openingtime set but without a closingtime set.
Ends: Thursday, 1 January 1970, 1:00 AM

This patch fixes this flaw in a way that it only shows the closingtime in addition to the openingtime if the closingtime is really set in the BBB activity instance. (PR#249)