routeMatch may lack commerce_order in inline form

Hi jsacksick,
Given that it's an inline form, I feel like we cannot assume that `getEntity()` will always return an order. The `getEntity()` method in my case returns `orderItem()`. One potential solution is to create an interface to verify if the callback_object form is an instance of this interface, and it's safe to use getOrder(), BTW Drupal\commerce_checkout\Plugin\Commerce\CheckoutFlow\MultistepDefault (what fresh install use) already uses the getOrder function Alternatively, we could pass the order as a configuration parameter and access it using `$configuration->['order']`.

This image shows a fresh install of the Kickstart, where the plugin form doesn't have getEntity() and doesn't implement the EntityFormInterface.

We could introduce a new interface, something like 'SupportsOrderEntity' with getOrder(), so that any form using an inline form payment gateway could implement it.

Hmmm, what if we removed them from CheckoutFlowInterface and let it extend the new interface?

  /**
   * Sets the current order.
   *
   * @param \Drupal\commerce_order\Entity\OrderInterface $order
   *   The order.
   *
   * @return $this
   */
  public function setOrder(OrderInterface $order): static;

  /**
   * Gets the current order.
   *
   * @return \Drupal\commerce_order\Entity\OrderInterface
   *   The current order.
   */
  public function getOrder();

Because not every form will have getPreviousStepId and getNextStepId (be checkoutflow form).
and we say if ($form_state->getFormObject() instanceof SupportsOrderEntity)?

I updated the logic. Let me know if that works for you.

Agreed!