fix 班级总课时为null时报错

mec-biz/src/main/java/com/ym/mec/biz/service/impl/ClassGroupServiceImpl.java

@@ -2148,7 +2148,8 @@ public class ClassGroupServiceImpl extends BaseServiceImpl<Integer, ClassGroup>
         if (Objects.isNull(classGroup)) {
             throw new BizException("班级不存在");
         }
-        classGroup.setTotalClassTimes(classGroup.getTotalClassTimes() + num);
+        int totalClassTimes = classGroup.getTotalClassTimes() == null ? 0 : classGroup.getTotalClassTimes();
+        classGroup.setTotalClassTimes(totalClassTimes+ num);
         classGroup.setUpdateTime(new Date());
 
         classGroupDao.update(classGroup);