"Delete selected Slots" runs into an error and won´t delete them.

Moodle Version 3.8

Scheduler Version: 3.7.0   |   2019120200

In the scheduler if I want to delete a few time slots, after selecting them I click on " delete selected slots" and it runs into following error: 

A required parameter (items) was missing

More information about this error
×Debug info:
Error code: missingparam
 
Stack trace: * line 494 of /lib/setuplib.php: moodle_exception thrown

• line 562 of /lib/moodlelib.php: call to print_error()
• line 237 of /mod/scheduler/teacherview.controller.php: call to required_param()
• line 141 of /mod/scheduler/teacherview.php: call to require_once()
• line 88 of /mod/scheduler/view.php: call to include()

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This also happened in previous Moodle Versions.