错误描述：
SQL语句中出现
Error evaluating expression ‘category.id != null and category.id != ”’. Cause: org.apache.ibatis.ognl.OgnlException: source is null for getProperty(null, “id”)
错误原因：
异常实际是在Mybatils执行映射处理的时候发生的，属性“XXX”找不到调用自己的所有者对象(NULL)，所以抛出异常
解决方式：
在对应的XXXMapper.xml中一步步判断字段（属性）所有者是否为空
从最初的字段所有者开始判断（先要判断category不能为空），避免空指针，才能继续下面的判断
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错误描述：
SQL语句中出现
Error evaluating expression ‘category.id != null and category.id != ”’. Cause: org.apache.ibatis.ognl.OgnlException: source is null for getProperty(null, “id”)