Problem with orchestra/testbench package:discover command inside my own package

[AlexisVS]

When i launch the command i have this result :

> @php ./vendor/bin/testbench package:discover --ansi

In MediaLibraryServiceProvider.php line 23:
                                                 
  Class name must be a valid object or a string  
                                                 

Script @php ./vendor/bin/testbench package:discover --ansi handling the post-autoload-dump event returned with error code 1

This is about the MediaObserver class I guess

MediaLibraryServiceProvider.php

public function boot()
    {
        $this->registerPublishables();

        $mediaClass = config('media-library.media_model');

        $mediaClass::observe(new MediaObserver());

        $this->loadViewsFrom(__DIR__.'/../resources/views', 'media-library');
    }

I don't know how can I solve this.

Someone can help me ? :)