Laravel - hasMany() and belongsTo() not working correctly for me in @foreach loop

I got following error when I try to use this on my page

Trying to get property of non-object

I have two tables:

cvicenis
  id
  title
  body
  category_id
categories
  id
  name

My Cviceni model:

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

class Cviceni extends Model
{
    public function category() 
    {
        return $this->belongsTo('App\Category', 'id');
    }
}

My Category model:

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

class Category extends Model
{
    public function cviceni()
    {
        return $this->hasMany('App\Cviceni', 'category_id');
    }
}

My controller:

public function index()
{
    $cviceni = Cviceni::all();
    return view('excercises.index', compact('cviceni'));
}

My index.blade:

@extends('layouts.app')

@section('content')
    <div class="row">
        <div class="col-sm-6 col-md-6 col-lg-3">
            <h1>Přehled cvičení</h1>
            @if(count($cviceni) > 0)
            <ul>
                @foreach ($cviceni as $c)
                    <li> </li>
                @endforeach
            </ul>
            @else
                <h2>There is nothing to display</h2>
            @endif
        </div><!-- end of column -->
    </div><!-- end of row -->
@endsection

If I use in my controller Cviceni::find(1) then I can get the value from $cviceni->category->name But not when I use a foreach loop. Can somebody help me please?

via Chebli Mohamed