notification: Fix NullPointerExceptions.
notification: Fix NullPointerExceptions.

NullPointerException could be thrown if lastEvent.notificationType is
null or/and event.newValue is null.

oolso/yobi,Limseunghwan/oss,bloodybear/yona,yona-projects/yona,bloodybear/yona,naver/yobi,doortts/fork-yobi,yona-projects/yona,bloodybear/yona,ChangsungKim/TestRepository01,ihoneymon/yobi,Limseunghwan/oss,doortts/forked-for-history,oolso/yobi,ihoneymon/yobi,doortts/fork-yobi,yona-projects/yona,doortts/forked-for-history,ihoneymon/yobi,naver/yobi,doortts/forked-for-history,naver/yobi,doortts/fork-yobi,Limseunghwan/oss,brainagenet/yobi,ahb0327/yobi,violetag/demo,brainagenet/yobi,ChangsungKim/TestRepository01,ahb0327/yobi,brainagenet/yobi,ahb0327/yobi,doortts/fork-yobi,yona-projects/yona,violetag/demo,bloodybear/yona,oolso/yobi