在null上调用成员函数save（）

I am using the following code

public function show()
{

    $id = Auth::user()->id;
    $usuario = User::find($id);
    $mascotin = Mascota::all();
    $mascota = Mascota::find($id);
    $mascota->save();

    $cant_mascota = Mascota::count();
    $cant_pregunta = Pregunta::count();

    return view('usuario.show',[
        'usuario' => $usuario,
        'mascotin' => $mascotin,
        'mascota' => $mascota,
        'cant_mascota' => $cant_mascota,
        'cant_pregunta' => $cant_pregunta,
    ]);
}

It gives me this error

 Symfony \ Component \ Debug \ Exception \ FatalThrowableError (E_ERROR).Call to a member function save() on null

User Model

public function mascotas(){
    return $this->hasMany('App\Mascota','user_id', 'id');

}

Mascota Model

public function usuario()
{   
    return $this->belongsTo('App\User', 'id','user_id');

}

Route

Route::get('/home', 'UserController@show')->name('home');

Hope you guys can help me, I'm new in laravel and I have like 1 day tring to solve this problem

$usuario = Auth::user();
$id = $usuario->id; // you already have user from Auth or Request, does not need to request database again
$mascotin = Mascota::all();
$mascota = $mascotin->find($id); // you can search in collection
//if you want to create Mascotin if it doesn't exists use Mascota::firstOrCreate(['id' => $id]);

if(!$mascota){
 throw new \Exception('Mascota not found', 404); //if $mascota is mandatory
}
$mascota->save(); // this does not have place here unless you are changing $mascota before that

$cant_mascota = $mascotin->count();
$cant_pregunta = Pregunta::count();

Also you should add auth middleware to this route. Only logged users should see it. I am not sure what "Mascota" means (it will be good to use english when you share your code) but it is not good to have the same id as user. Better use relationships.