Laravel 5.7 - Undefined offset: 1

I'm new in Laravel and it is my first question. I have 3 tables:

categories: id, name (at the moment 2 items)

variants: id, name

category_variant: id, category_id, variant_id; <-- Every variant has 1 or 2 categories

In the VariantController I have following code:

public function edit($id)
{
    $variant = Variant::where('id', $id)->with('categories')->first();
    $categories = Category::all();

    return view('admin.variant.edit', compact('variant', 'categories'));
}

In the edit.blade.php I have following html:

@foreach ($categories as $key=>$category)
   <div class="form-group form-float">
   @if (isset($variant->categories[$key]->pivot->category_id)) <-- I think here is the problem
      <input type="checkbox" id="wb" class="filled-in" name="wb" value=""  >
      <label for="wb"></label>
   @else
      <input type="checkbox" id="wb" class="filled-in" name="wb" value="">
      <label for="wb"></label>
   @endif
   </div>
@endforeach

I want to know which category was checked in the checkbox. If the variant has all 2 categories everthing is ok but if the user has chosen only one category I get an error

Undefined offset: 1 (View: /shui/resources/views/admin/variant/edit.blade.php)

How can I solve this problem? Thanks in advance Dimi