fix(ras-acc): add check to prevent JS error in variation modal

[laurelfulford]

All Submissions:

• [x] Have you followed the Newspack Contributing guideline?
• [x] Does your code follow the WordPress' coding standards and VIP Go coding standards?
• [x] Have you checked to ensure there aren't other open Pull Requests for the same update/change?

Changes proposed in this Pull Request:

I noticed this randomly while testing another PR: the changes I made to keep modals open while redirecting broke the ability to close the variation modal.

How to test the changes in this Pull Request:

1. On epic/ras-acc, set up a Checkout Button block with a variable product where the reader can pick the variation.
2. On the front end, click the Checkout Button block to open the variation picker.
3. Click the close button; note it doesn't close and throws a JS error.
4. Apply this PR and run npm run build.
5. Repeat steps 2-3 and confirm that the modal closes as expected.
6. Repeat a couple of the tests from https://github.com/Automattic/newspack-blocks/pull/1855 and confirm that the modal-not-closing-when-redirecting behaviour still works as expected.

Other information:

• [x] Have you added an explanation of what your changes do and why you'd like us to include them?
• [ ] Have you written new tests for your changes, as applicable?
• [ ] Have you successfully ran tests with your changes locally?

[laurelfulford]

Closing; it looks like this is already fixed in https://github.com/Automattic/newspack-blocks/pull/1861 😆