Server error function get () on null

The server log reported the following error:

Notice :

  

Undefined variable: app in C: \ xxxx \ api \ v1 \ routers \ Schedule.php on line   28

Fatal error :

  

Call to a member function get () on null in   /home/xxx/public_html/projects/xxx/api/v1/routers/Schedule.php on line   22

The code is organized as follows:

Route class:

    use xxx\API\API_Schedule;
    use xxx\model\Schedule;
    use Slim\Http\Request;
    use Slim\Http\Response;

       $app->get('/schedule', function (Request $request, Response $response, $args ){
               $delivery_id = $request->getParam('deliveryman_id');

           $api = new API_Schedule();

           $resposta = $api->getSchedules($delivery_id);

           $response->write($resposta);
        });
    $app->post('/schedule/deny/{id}', function(Request $request, Response $response, $args){
          $deliveryman_id = $request->getParam('deliveryman_id');

          $schedule_id = $args['id'];

          $api = new API_Schedule();

          $response->write($api->denySchedule($deliveryman_id, $schedule_id));
        });

    $app->post('/schedule/accept/{id}', function(Request $request,Response $response, $args){
        $deliveryman_id = $request->getParam('deliveryman_id');
        $schedule_id = $args['id'];

        $api = new API_Schedule();

        $response->write($api->acceptSchedule($deliveryman_id, $schedule_id));
    });


    $app->post('/schedule/accept', function(Request $request,Response $response, $args){
        $deliveryman_id = $request->getParam('deliveryman_id');
        $api = new API_Schedule();

        $data = $api->scheduleAccepted($deliveryman_id);

        if($data!= false){
            $response->write($data);
        }
    });

$app->post('/schedule', function (Request $request, Response $response, $args ){
        $schedule = new Schedule();
        $schedule->name = $request->getParam('name');
        $schedule->date_to = $request->getParam('date_to');
        $schedule->min_deliveryman = $request->getParam('min_deliveryman');
        $schedule->max_deliveryman = $request->getParam('max_deliveryman');
        $schedule->status = 0;
        $schedule->client_id = $request->getParam('client_id');
        $schedule->daily_price = $request->getParam('daily_price');
        $schedule->delivery_price = $request->getParam('delivery_price');

        $api = new API_Schedule();

        $response->write($api->addNewSchedule($schedule));
    });

Index class:

<?php

use Slim\App;
require (__DIR__.'/../../vendor/autoload.php');

session_start();

$app = new App();

include (__DIR__.'/routers/Schedule.php');

$app->run();

The error is strange because this is not the only existing route, however it is the only one returning this error, thanks in advance.