Issue #3249988 by Dave Kopecek, idebr: Error: Call to a member function toUrl() on null.

  public function toUrl($rel = 'canonical', array $options = []) {

    // For better UX, the collection URL of invoices that are created for a

    // single order should be the list of invoices for that specific order.

    $invoice_collection_url = NULL;

    if ($rel === 'collection' && count($this->get('orders')) === 1) {

      $order = $this->get('orders')->first()->entity;

      if ($order) {

        if ($this->bundle() === 'default') {

        $invoice_collection_url = $order->toUrl('invoices', $options);

          return $order->toUrl('invoices', $options);

        }

        elseif ($this->bundle() === 'credit_memo') {

        $invoice_collection_url = $order->toUrl('credit-memos', $options);

          return $order->toUrl('credit-memos', $options);

        }

      }

    }

    return $invoice_collection_url ?? parent::toUrl($rel, $options);

    return parent::toUrl($rel, $options);

  }

  /**