bbs模块发布有错误提示“An uncaught Exception was e

从会员中心发布bbs模块的内容的时候，出现错误提示，但是时间上已经发布成功，关闭错误提示后也能正常访问发布的内容。

从后台发布也没问题。请教下怎么破？

错误提示如下：

An uncaught Exception was encountered

Type: Error

Message: Call to a member function where() on null

Filename: /www/wwwroot/xxxxxx/diy/module/bbs/models/Content_model.php

Line Number: 31

Backtrace:

File: /www/wwwroot/xxxxxx/diy/branch/fqb/C_Model.php

Line: 395

Function: _add_content

File: /www/wwwroot/xxxxxx/diy/branch/fqb/D_Member_Home.php

Line: 249

Function: add

File: /www/wwwroot/xxxxxx/diy/Init.php

Line: 72

Function: require

File: /www/wwwroot/xxxxxx/index.php

Line: 49

Function: require