New Payment IDs breaks the module

[wouter-toppy]

Thank you for creating this pull request! To make the best use of your and our time we created this checklist to get the best possible pull requests:

• [x] The code is working on a plain Magento 2 installation.
• [x] The code follows the PSR-2 code style.
• [x] When an exception or error is logged the message is accompanied with some context, eg: Error when trying to get the payment status:
• [x] Contains tests for the changed/added code (great if so but not required).
• [x] I have added a scenario to test my changes.

This PR touches code in the following areas (Check what is applicable):

Frontend

• [ ] Shopping cart
• [x] Checkout
• [ ] Totals
• [ ] Payment methods

Backend

• [x] Configuration
• [ ] Order grid
• [ ] Order view
• [ ] Invoice view
• [ ] Credit memo view
• [ ] Shipment view
• [ ] Email sending

Order Processing (Mollie communication)

• [ ] Creating the order
• [x] Invoicing the order
• [ ] Shipping the order
• [ ] Refunding (credit memo) the order

Please describe the bug/feature/etc this PR contains: Fixes: #669

[wouter-toppy]

@Frank-Magmodules

[mollierobbert]

This will work for now, but the numeric prefix will increase to more than a single digit as the dataset grows further. (A few years down the road.)

Sorry, I see this MR was just a hot fix. Thanks for the support!

The fix that Magmodules released now is compatible with multi-digit prefixes as well.