Fix the issue with "Shipping address is not set" exception

[dmytro-ch]

The exception is being thrown instead of redirecting the customer to New Shipping Address form.

Description

The issues appeared because of the interrupted shipment assignment process.
As result:

1. The "Shipping address is not set" exception (Magento\Framework\Exception\StateException) is not caught and the customer is not redirected to multishipping/checkout_address/newShipping page.
2. The incorrectly assigned record appears in quote_address table due to the interrupted process of quote saving by Magento\Framework\Exception\StateException. It causes the additional error on Shopping Cart page, after performing the steps mentioned in Manual testing scenarios. (Fixed in: 6f88af5)

Fixed Issues

1. "Shipping address is not set" exception in Multishipping Checkout. #16555: "Shipping address is not set" exception in Multishipping Checkout.
2. "Shipping address is not set" exception in Multishipping Checkout. #16555: "SQLSTATE[23000]: Integrity constraint violation" when trying to access Shopping Cart.

Manual testing scenarios

1. Log In as Customer.
2. Add Product to Cart.
3. Proceed to Checkout.
4. Fill Shipping Address data, click Next (do not place Order)
5. Return to the Storefront.
6. Go to the Shopping Cart.
7. Click Check Out with Multiple Addresses. (Error)
8. Go to the Shopping Cart page again. (Error)

Contribution checklist

• Pull request has a meaningful description of its purpose
• All commits are accompanied by meaningful commit messages
• All new or changed code is covered with unit/integration tests (if applicable)
• All automated tests passed successfully (all builds on Travis CI are green)

[magento-engcom-team]

Hi @dmytro-ch. Thank you for your contribution
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[rogyar]

Hi @dmytro-ch. Thank you for your collaboration. That's great that we have a fix, but we should slightly improve it. Let me share my thoughts. The apply method has nothing to do with the redirection. It's just a service method that sets a shipping method. So, that's absolutely normal that it throws the exception when there's no shipping address set. Other 3rd party extensions or custom implementations might expect this behavior and it's logical. So, it's not good for us to change logic on such a low level of implementation.
To fix the redirection issue, we need to find a logical part that is responsible for the redirection and make changes there. I.e. catch the StateException (or bring the StateException to this point if it's being caught somewhere else) and perform the corresponding checks/actions.

[rogyar]

@dmytro-ch are you going to proceed with this one?

[dmytro-ch]

@rogyar I will update this PR till the end of the week. Thank you!

[dmytro-ch]

@rogyar I've updated the PR. Could you please check the result?
Thank you!

[magento-engcom-team]

Hi @rogyar, thank you for the review.
ENGCOM-2704 has been created to process this Pull Request

[magento-engcom-team]

Hi @dmytro-ch. Thank you for your contribution.
We will aim to release these changes as part of 2.2.7.
Please check the release notes for final confirmation.

Please, consider to port this solution to 2.3 release line.
You may use Porting tool to port commits automatically.