Added null stirng check.
Added null stirng check.

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