check null

mooctest-site-server/src/main/java/cn/iselab/mooctest/site/web/logic/impl/GroupLogicImpl.java

@@ -205,7 +205,7 @@ public class GroupLogicImpl extends BaseLogic implements GroupLogic {
     @Override
     public GroupVO joinGroup(HttpServletRequest request, long userId, GroupVO groupVO) {
 
-        if ((groupVO.getCaptcha().equals("summer")) || CaptchaUtils.verifyPictureCaptcha(request, groupVO.getCaptcha())) {
+        if ( groupVO.getCaptcha()!=null && ( groupVO.getCaptcha().equals("summer") || CaptchaUtils.verifyPictureCaptcha(request, groupVO.getCaptcha()))) {
             String managerName = groupVO.getManagerName();
             Group group = groupService.getGroup(groupVO.getId());
             User user=userService.findByUserId(group.getOwnerId());