PhabricatorSearchTextField.php's parent class PhabricatorSearchField.php defines an (empty) protected function validateControlValue($value).
Override this function in PhabricatorSearchTextField.php by disallowing arrays (as the only data type which cannot be casted into a text value).

Closes T15714

Apply D25518; then manually edit the code in ManiphestTaskSearchEngine.php by defining an array and passing that array to the Page Size text field via id(new PhabricatorSearchTextField())->setDefaultValue($empty_array), then go to /maniphest/query/advanced/ and see the error message.