Errors in codeigniter-restserver library

I want to use restful in my ci 3.03 application:

1. I found this tutplus tutorial
2. I downloaded codeigniter-restserver-master.zip file and copied Format.php and REST_Controller.php(@version 3.0.0) files into /application/libraries/REST directory

3. I created control application/controllers/api/Users.php :

   require_once("application/libraries/REST/REST_Controller.php");
   require_once("application/libraries/REST/Format.php");
   
   class Users extends REST_Controller
   {
   //protected $rest_format = 'json';
   
     function users_get()
     {
         //$users = $this->user_model->get_all();
         $filter_username= $this->get('filter_username');
         $filter_user_group= $this->get('filter_user_group');
         $filter_active= $this->get('filter_active');
         $sort= $this->get('sort');
         $sort_direction= $this->get('sort_direction');
   
         //, $filter_user_group, $filter_active, $sort, $sort_direction
         $users_list = $this->muser->getUsersList(false, ''/*, $filter_username, $filter_user_group, $filter_active, $sort, $sort_direction, ''*/);
         echo '<pre>'.count($users_list).'::$users_lists::'.print_r($users_list,true).'</pre>';
   
   
         if($users_list)
         {
             $this->response($users, 200);
         }
   
         else
         {
             $this->response(NULL, 404);
         }
     }
   

AND RUNNING URL http://local-ci3.com/api/users I got many errors:

A PHP Error was encountered

Severity: Notice
Message: Undefined property: Users::$format
Filename: REST/REST_Controller.php
Line Number: 734
Backtrace:
File: /mnt/diskD_Work/wwwroot/ci3/application/libraries/REST/REST_Controller.php
Line: 734
Function: _error_handler
File: /mnt/diskD_Work/wwwroot/ci3/application/libraries/REST/REST_Controller.php
Line: 649
Function: response
File: /mnt/diskD_Work/wwwroot/ci3/index.php
Line: 292
Function: require_once

A PHP Error was encountered
Severity: Notice
Message: Undefined property: Users::$format
Filename: REST/REST_Controller.php
Line Number: 752
Backtrace:
File: /mnt/diskD_Work/wwwroot/ci3/application/libraries/REST/REST_Controller.php
Line: 752
Function: _error_handler
File: /mnt/diskD_Work/wwwroot/ci3/application/libraries/REST/REST_Controller.php
Line: 649
Function: response
File: /mnt/diskD_Work/wwwroot/ci3/index.php
Line: 292
Function: require_once

Actually I wanted to get some workable library to help me with REST api creation. I think that is preferable way istead of making from zero. But is this library not workable or does it needs for some fixing? Sorry, what I missed is if this library only for ci 2?

I made search on this forum and found such hint :

I have the same problem when I load both Format.php and Rest_Controller.php into a controller. After have a quick glance at Format.php, it appears to be a standalone format conversion helper. Try to just load Rest_Controller.php and see if your problem goes away.

I commented line

//require_once("application/libraries/REST/Format.php");

in my controller, but I still get errors like : Message: Undefined property: Users::$format. I tried to review code of this library and see that invalid block when data are converted to json format, line 731-757 :

elseif ($data !== NULL)
{
    // If the format method exists, call and return the output in that format
    if (method_exists($this->format, 'to_' . $this->response->format))
    {
        // Set the format header
        $this->output->set_content_type($this->_supported_formats[$this->response->format], strtolower($this->config->item('charset')));
        $output = $this->format->factory($data)->{'to_' . $this->response->format}();

        // An array must be parsed as a string, so as not to cause an array to string error
        // Json is the most appropriate form for such a datatype
        if ($this->response->format === 'array')
        {
            $output = $this->format->factory($output)->{'to_json'}();
        }
    }
    else
    {
        // If an array or object, then parse as a json, so as to be a 'string'
        if (is_array($data) || is_object($data))
        {
            $data = $this->format->factory($data)->{'to_json'}();
        }

        // Format is not supported, so output the raw data as a string
        $output = $data;
    }
}

If I tried to commented this block, but get error

Message: Array to string conversion

Looks like data are not converted in this case...

Is is possible to fix these errors? Or can you, please, to tell me advice some codeigniter 3 REST api workable library with similar interface like library above?

Thanks!