WCF::handleException() Unhandled exception: Call to a member function canEnter() on null

#0 /var/www/vhosts/heimkinder-forum.de/httpdocs/v4x/lib/system/attachment/PostAttachmentObjectType.class.php(31): wbb\data\post\Post->canRead()
#1 [internal function]: wbb\system\attachment\PostAttachmentObjectType->canDownload('449769')
#2 /var/www/vhosts/heimkinder-forum.de/httpdocs/v4x/wcf/lib/data/attachment/Attachment.class.php(90): call_user_func(Array, '449769')
#3 /var/www/vhosts/heimkinder-forum.de/httpdocs/v4x/wcf/lib/data/attachment/Attachment.class.php(48): wcf\data\attachment\Attachment->getPermission('canDownload')
#4 /var/www/vhosts/heimkinder-forum.de/httpdocs/v4x/wcf/lib/page/AttachmentPage.class.php(95): wcf\data\attachment\Attachment->canDownload()
#5 /var/www/vhosts/heimkinder-forum.de/httpdocs/v4x/wcf/lib/page/AbstractPage.class.php(176): wcf\page\AttachmentPage->checkPermissions()
#6 /var/www/vhosts/heimkinder-forum.de/httpdocs/v4x/wcf/lib/page/AttachmentPage.class.php(146): wcf\page\AbstractPage->show()
#7 /var/www/vhosts/heimkinder-forum.de/httpdocs/v4x/wcf/lib/page/AbstractPage.class.php(86): wcf\page\AttachmentPage->show()
#8 /var/www/vhosts/heimkinder-forum.de/httpdocs/v4x/wcf/lib/system/request/Request.class.php(58): wcf\page\AbstractPage->__run()
#9 /var/www/vhosts/heimkinder-forum.de/httpdocs/v4x/wcf/lib/system/request/RequestHandler.class.php(108): wcf\system\request\Request->execute()
#10 /var/www/vhosts/heimkinder-forum.de/httpdocs/v4x/index.php(10): wcf\system\request\RequestHandler->handle('wbb')
#11 {main}