Problem when removing an Element

[AnneGF]

If this plugin is active and one wants to remove an Element ($PATH/admin/settings/edit-item-type-elements) and it is not empty (at least one item has a value for this element), an error occur:

Zend_Db_Statement_Mysqli_Exception

    Mysqli statement execute error : Column 'part_of' cannot be null

Zend_Db_Statement_Mysqli_Exception: Mysqli statement execute error : Column 'part_of' cannot be null in /var/www/html/application/libraries/Zend/Db/Statement/Mysqli.php:215
Stack trace:
#0 /var/www/html/application/libraries/Zend/Db/Statement.php(303): Zend_Db_Statement_Mysqli->_execute(Array)
#1 /var/www/html/application/libraries/Zend/Db/Adapter/Abstract.php(480): Zend_Db_Statement->execute(Array)
#2 /var/www/html/application/libraries/Omeka/Db.php(80): Zend_Db_Adapter_Abstract->query('\n            IN...')
#3 /var/www/html/plugins/HistoryLog/HistoryLogPlugin.php(434): Omeka_Db->__call('query', Array)
#4 /var/www/html/application/libraries/Omeka/Plugin/Broker.php(154): HistoryLogPlugin->hookBeforeDeleteElement(Array)
#5 /var/www/html/application/libraries/Omeka/Record/AbstractRecord.php(301): Omeka_Plugin_Broker->callHook('before_delete_e...', Array)
#6 /var/www/html/application/libraries/Omeka/Record/AbstractRecord.php(578): Omeka_Record_AbstractRecord->runCallbacks('beforeDelete')
#7 /var/www/html/application/controllers/SettingsController.php(159): Omeka_Record_AbstractRecord->delete()
#8 /var/www/html/application/libraries/Zend/Controller/Action.php(516): SettingsController->editItemTypeElementsAction()
#9 /var/www/html/application/libraries/Zend/Controller/Dispatcher/Standard.php(308): Zend_Controller_Action->dispatch('editItemTypeEle...')
#10 /var/www/html/application/libraries/Zend/Controller/Front.php(954): Zend_Controller_Dispatcher_Standard->dispatch(Object(Zend_Controller_Request_Http), Object(Zend_Controller_Response_Http))
#11 /var/www/html/application/libraries/Zend/Application/Bootstrap/Bootstrap.php(105): Zend_Controller_Front->dispatch()
#12 /var/www/html/application/libraries/Zend/Application.php(384): Zend_Application_Bootstrap_Bootstrap->run()
#13 /var/www/html/application/libraries/Omeka/Application.php(73): Zend_Application->run()
#14 /var/www/html/admin/index.php(28): Omeka_Application->run()
#15 {main}