Error: Call to a member function can_read() on null

Aus dem error_log - das passiert bei Auswahl eines Monats in der Liste der Studenzettel:

Error: Call to a member function can_read() on null in public/plugins_packages/virtUOS/Stundenzettel/views/timesheet/timesheet.php:28
Stack trace:
#0 vendor/flexi/lib/php_template.php(46): require()
#1 vendor/flexi/lib/template.php(157): Flexi_PhpTemplate->_render()
#2 vendor/trails/trails.php(764): Flexi_Template->render()
#3 vendor/trails/trails.php(724): Trails_Controller->render_template('timesheet/times...', Object(Flexi_PhpTemplate))
#4 public/plugins_packages/virtUOS/Stundenzettel/controllers/timesheet.php(122): Trails_Controller->render_action('timesheet')
#5 vendor/trails/trails.php(557): TimesheetController->select_action('108')
#6 app/controllers/studip_controller.php(113): Trails_Controller->perform('select/108')
#7 vendor/trails/trails.php(154): StudipController->perform('select/108')
#8 vendor/trails/trails.php(121): Trails_Dispatcher->map_uri_to_response('timesheet/selec...')
#9 lib/plugins/core/StudIPPlugin.class.php(166): Trails_Dispatcher->dispatch('timesheet/selec...')
#10 public/plugins.php(54): StudIPPlugin->perform('timesheet/selec...')