我用categoryId=0测试，服务器错误500，但是用categoryId=100001测试，返回正确

报错信息如下：

root cause

java.lang.NullPointerException
com.tmall.service.impl.CategoryServiceImpl.selectCategoryAndChildrenById(CategoryServiceImpl.java:82)
com.tmall.controller.backend.CategoryManageController.getCategoryAndDeepChildrenCategory(CategoryManageController.java:84)

以下是是我的源代码：

public ServerResponse selectCategoryAndChildrenById(Integer categoryId) {
   Set<Category> categorySet = Sets.newHashSet();
   findChildCategory(categorySet, categoryId);

   List<Integer> categoryIdList = Lists.newArrayList();
   if (categoryId != null) {
       for (Category categoryItem : categorySet){
           categoryIdList.add(categoryItem.getId());
       }
   }
   return ServerResponse.createBySuccess(categoryIdList);
}

//递归算法，算出子节点
private Set<Category> findChildCategory(Set<Category> categorySet, Integer categoryId) {
   Category category = categoryMapper.selectByPrimaryKey(categoryId);
   categorySet.add(category);

   List<Category> categoryList = categoryMapper.selectCategoryChildrenByParentId(categoryId);
   for (Category categoryItem :
           categoryList) {
       findChildCategory(categorySet, categoryItem.getId());
   }
   return categorySet;
}