报错 PHP Error: Call to a member function getMessage () on null in Psy Shell code on line 4

这个报错是什么问题呢？EasySmsServiceProvider.php代码如下：

<?php

namespace App\Providers;

use Overtrue\EasySms\EasySms;
use Illuminate\Support\ServiceProvider;

class EasySmsServiceProvider extends ServiceProvider
{
    /**
     * Register services.
     *
     * @return void
     */
     public function register()
     {
         $this->app->singleton(EasySms::class, function ($app) {
             return new EasySms(config('easysms'));
         });

         $this->app->alias(EasySms::class, 'easysms');
     }

    /**
     * Bootstrap services.
     *
     * @return void
     */
    public function boot()
    {
        //
    }
}

tinker报错信息如下：

$ php artisan tinker
Psy Shell v0.9.9 (PHP 7.1.16 — cli) by Justin Hileman
>>> $sms = app('easysms');
=> Overtrue\EasySms\EasySms {#3209}
>>> try {
...     $sms->send(13888853427, [
...         'content'  => ' Lbbs         1234              ',
...     ]);
... } catch (\Overtrue\EasySms\Exceptions\NoGatewayAvailableException $exception) {
...     $message = $exception->getException('yunpian')->getMessage();
...     dd($message);
... }
PHP Error:  Call to a member function getMessage() on null in Psy Shell code on line 4