“消息：在助手中调用null上的成员函数post（）

I get an error in controller and i added construct func and i add get_instance in helper but i get this error .

Helper added autoload.php

I've tried looking stackoverflow and I couldn't find anything

//in helper
function postvalue($name){

    $ci =& get_instance();
    return $ci->input->post($name);
}

//in controller

          $isexist = Process_admin::find([
            "root_username"=>postvalue("username"),
          "root_password"=>md5(postvalue("password"))

        ]);

Error message :

I was waiting for the output as a post from the dire but I get an error

You can directly access name value by $name in helper function