fixed fatal error in notification center util function (fixes #2756)
fixed fatal error in notification center util function (fixes #2756)

tejima/OP3PKG,nise-nabe/ppcon-sns,nise-nabe/ppcon-sns,openpne/OpenPNE3,openpne/OpenPNE3,openpne/OpenPNE3,tejima/OP3PKG,openpne/OpenPNE3