Laravel Artisan event:generate command throwing a Fatal error

I'm following the official documentation for Laravel 5.7 on the events registration and generation: https://laravel.com/docs/5.7/events#generating-events-and-listeners

I have an EventServiceProvider with the following events defined:

/**
 * The event handler mappings for the application.
 *
 * @var array
 */
protected $listen = [
    'App\Events\FormBeforeCreate' => [
        'App\Listeners\WebhookBeforeCreate',
    ],
    'App\Events\FormAfterCreate' => [
        'App\Listeners\NotifyAfterCreate',
        'App\Listeners\WebhookAfterCreate',
    ],
    'App\Events\FormBeforeUpdate' => [
        'App\Listeners\WebhookBeforeUpdate',
    ],
    'App\Events\FormAfterUpdate' => [
        'App\Listeners\NotifyAfterUpdate',
        'App\Listeners\WebhookAfterUpdate',
    ],
    'App\Events\FormBeforeDelete' => [
        'App\Listeners\WebhookBeforeDelete',
    ],
    'App\Events\FormAfterDelete' => [
        'App\Listeners\NotifyAfterDelete',
        'App\Listeners\WebhookAfterDelete',
    ],
    'App\Events\FormBeforeSave' => [
        'App\Listeners\WebhookBeforeSave',
    ],
    'App\Events\FormAfterSave' => [
        'App\Listeners\NotifyAfterSave',
        'App\Listeners\WebhookAfterSave',
    ],
];

The error:

When I run the command php artisan event:generate I get the following error:

PHP Fatal error:  Call to a member function listens() on null in /app/vendor/laravel/framework/src/Illuminate/Foundation/Console/EventGenerateCommand.php on line 35

                                                           
  [Symfony\Component\Debug\Exception\FatalErrorException]  
  Call to a member function listens() on null

According to the doc, it should do this:

This command will generate any events or listeners that are listed in your EventServiceProvider. Events and listeners that already exist will be left untouched

I don't understant what I've missed since I didn't find any similar error by searching the web

via Chebli Mohamed