Trying to get property of non-object

Hello,

My head is spinning and I cannot figure out why I'm getting the Trying to get property of non-object error.

This is the error:

PHP Code:

A PHP Error was encountered

Severity: Notice

Message: Trying to get property of non-object

Filename: models/Crud_model.php

Line Number: 924

Backtrace:

File: /Users/Public/Git/Acme/acme.dev/application/models/Crud_model.php
Line: 924
Function: _error_handler

File: /Users/Public/Git/Acme/acme.dev/application/models/Crud_model.php
Line: 968
Function: is_added_by

File: /Users/Public/Git/Acme/acme.dev/application/views/back/includes_bottom.php
Line: 115
Function: is_sale_of_vendor

File: /Users/Public/Git/Acme/acme.dev/application/views/back/index.php
Line: 32
Function: include

File: /Users/Public/Git/Acme/acme.dev/application/controllers/Vendor.php
Line: 32
Function: view

File: /Users/Public/Git/Acme/acme.dev/index.php
Line: 292
Function: require_once 


And here comes the method from my Model:

PHP Code:

   function is_added_by($type,$id,$user_id,$user_type = 'vendor') {
       $added_by = json_decode($this->db->get_where($type,array($type.'_id'=>$id))->row()->added_by,true);  
        if($user_type == 'admin'){
            $user_id = $added_by['id'];
        }
        if($added_by['type'] == $user_type && $added_by['id'] == $user_id){
            return true;
        } else {
            return false;
        }
    } 


The line 924 is the one with json_decode.

Any help is appreciated!