Replace next function: public function having_cap($cap) { $contextswithcap = array(); foreach ($this->allcontexts as $context) { if (has_capability($cap, $context)) { $contextswithcap[] = $context; } } return $contextswithcap; }   with: public function having_cap($cap) { $contextswithcap = array(); foreach ($this->allcontexts as $context) { if ($context!=NULL && has_capability($cap, $context)) { $contextswithcap[] = $context; } } return $contextswithcap; }