issue: Fix error on deleting issue.
issue: Fix error on deleting issue.

Fix an error while trying to delete an issue whose miletone is not set.

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