Fatal error occures sometimes when submitting webform with documents

[ruudgithub]

Occasionally the following fatal error message occurs when users submit a webform with a document:

Trying to get property of non-object in WebformCiviCRMDocuments->saveFileAsDocument() (line 185 of /var/www/drupal/sites/all/modules/civicrm_extensions/org.civicoop.documents/drupal/webform_civicrm_documents/WebformCiviCRMDocuments.inc).

Fatal error: Call to a member function realpath() on boolean in /var/www/drupal/sites/all/modules/civicrm_extensions/org.civicoop.documents/drupal
/webform_civicrm_documents/WebformCiviCRMDocuments.inc on line 186

Drupal 7.5.9
CiviCRM 4.4.8
org.civicoop.documents 1.8