Php – Call to a member function isAdmin() on null in laravel

I want to basic authenticate if User is Admin then Request next otherwise redirect to homepage

User.php:

<?php

namespace App;

use Illuminate\Foundation\Auth\User as Authenticatable;

class User extends Authenticatable
{
    /**
     * The attributes that are mass assignable.
     *
     * @var array
     */
    protected $fillable = [
        'name', 'email', 'password','role_id','is_active','photo_id',
    ];

    /**
     * The attributes that should be hidden for arrays.
     *
     * @var array
     */
    protected $hidden = [
        'password', 'remember_token',
    ];

    public function role(){
        return $this->belongsTo('App\Role');
    }

    public function photo(){
        return $this->belongsTo('App\Photo');
    }

    public function isAdmin(){
        if ($this->role()->name=="administrator"){
            return true;
        }
        return false;
    }
}

last function is isAdmin()

Admin.php (this is middleware):

<?php

namespace App\Http\Middleware;

use Closure;
use Illuminate\Support\Facades\Auth;

class Admin
{
    /**
     * Handle an incoming request.
     *
     * @param  \Illuminate\Http\Request  $request
     * @param  \Closure  $next
     * @return mixed
     */
    public function handle($request, Closure $next)
    {
        if (Auth::check()){

            if (Auth::user()->isAdmin()){

                return $next($request);

            }
        }

        return redirect('/');
    }
}

routes.php:

<?php

Route::get('/', function () {
    return view('welcome');
});


Route::get('/admin',function(){
    return view('admin.index');
});
Route::group(['middleware'=>'admin'],function(){

    Route::resource('/admin/users','AdminUsersController');

});




Route::auth();

Route::get('/home', 'HomeController@index');

I get the following error:

FatalErrorException in Admin.php line 21:
Call to a member function isAdmin() on null

I also added 'admin' =>\App\Http\Middleware\Admin::class, in kernel.php and imported the class in Admin.php.