Error Number: 1048

I do not know why I get this error, can someone help me?

A Database Error Occurred
Error Number: 1048
Column 'file_id' cannot be null
INSERT INTO `bse_products_files` (`product_id`, `file_id`) VALUES (91, NULL)
Filename: /home/c3fctk/public_html/localhost/models/digital_product_model.php
Line Number: 94

and here is the line 94

• function associated($file_name, $product_id, $user_id)
  
•     {
  
•         $file_id = $this->db->where('title', $file_name)->where('user_id', $user_id)->get('digital_products')->row();
  
•         $this->db->insert('products_files', array(
  
•             'product_id' => $product_id,
  
•             'file_id' => $file_id->id
  
•         ));
  
•     }
  

 line 94 

 'file_id' => $file_id->id