call to a member function store () on null Laravel php?

Cordial greetings fellow, I have the following problem, I have an application that lets you upload images laravel to use this js dropzone, when loading the image into the form I get the following error:

and pressing the load button displays the following error:

But the image is saved in the folder that I tell you, but the record is not saved in the database.

This is the submission form:

@extends('layouts.layout')

<script
src="https://code.jquery.com/jquery-3.3.1.min.js"
integrity="sha256-FgpCb/KJQlLNfOu91ta32o/NMZxltwRo8QtmkMRdAu8="
crossorigin="anonymous"></script>


<link rel="stylesheet" href="https://cdnjs.cloudflare.com/ajax/libs/dropzone/5.4.0/dropzone.css">



@section('contenido')

<div>

<p>Album: {{ $foto->id }}</p>

</div>
<form action="{{ route('fotos') }}" method="POST" enctype="multipart/form-data">

        {{ csrf_field() }}  

<div class="form-group">

{{-- Este div es para cargar las imagenes --}}
<div class="dropzone">



</div>

<input type="text" name="id" value="{{ $foto->id }}" hidden>

<div class="form-group">

        <button type="submit" class="btn btn-primary">Cargar</button>

    </div>
</form>
</div>

@stop


@push('scripts')

<script src="https://cdnjs.cloudflare.com/ajax/libs/dropzone/5.4.0/min/dropzone.min.js"></script>






<script >




var myDropzone = new Dropzone('.dropzone', {

// url por donde pasaremos las imagenes del post
url: '{{route("fotos")}}',

// con esta sentencia le decimos a dropzone que solo acepte archivos de imagen y con el * le decimos que acepte imagenes con cualquier extension
acceptedFiles: 'image/*',

// con esta sentencia le definimos a dropzone la restriccion de tamaño del archivo a subir en Mega Bytes
maxFilesize: 2,

// con esta sentencia restringimos la cantidad de archivos que se pueden subir
// maxFiles: 1,

paramName: 'photo',



headers: {

    // verificacion del token
    'X-CSRF-TOKEN': '{{ csrf_token() }}'

},

// esta sentencia cambia el mensaje por defecto que aparece en la caja de dropzone
dictDefaultMessage : 'Arrastra las imagenes aqui para subirlas'

});

myDropzone.on('error', function(file , res){

console.log(res);

});

// con esta sentencia le decimos a dropzone que no se active automaticamente, solo cuando le demos clic
Dropzone.autoDiscover = false;

</script>


@endpush

This is the route it goes through:

Route::post('fotos','FotoController@upload')->name('fotos');

This is the upload function of the FotoController to which the form data arrives:

 public function upload(Request $request)
    {

    $foto = request()->file('photo')->store('fotos', 'public'); //Si comento esta linea

    $fotos = new Foto;

    $fotos->url = $foto; // y aqui agrego una url manualmente, el registro si se guarda en la base de datos con el id del album que llega por el formulario

    $fotos->album_id = $request->id;

    $fotos->user_id = auth()->user()->id;

    $fotos->save();



     return 'imagen guardada';

    }

I also attach the code of the migrations and models of the album table and photos:

Migration of the table photos:

 public function up()
    {
        Schema::create('fotos', function (Blueprint $table) {
            $table->increments('id');
            $table->string('url');
            $table->unsignedInteger('album_id');
            $table->unsignedInteger('user_id');

            $table->timestamps();
        });
    }

Photo model:

<?php

namespace App;

use Illuminate\Database\Eloquent\Model;
use App\Album;

class Foto extends Model
{

    protected $table = 'fotos';

    protected $fillable = ['url','album_id','user_id'];

    protected $guard = ['id'];



    public function Foto()
    {
     return $this->belongsTo(Album::class, 'album_id');
    }
}

Migration of the albums table:

 public function up()
    {
        Schema::create('albums', function (Blueprint $table) {
            $table->increments('id');
            $table->string('nombre_album');
            $table->integer('user_id');
            $table->timestamps();
        });
    }

Album Model:

<?php

namespace App;

use Illuminate\Database\Eloquent\Model;
use App\User;
use App\Foto;

class Album extends Model
{
  protected $table = 'albums';

  protected $fillable = ['nombre_album','user_id'];

  protected $guard = ['id'];

  public function User()
  {
   return $this->belongsTo(User::class, 'user_id');
  }

  public function fotos()
  {
   return $this->hasMany(Foto::class);
  }
}

What can be wrong here?