Prerequisites:

1. Configure payment method
2. Configure shipping method

Steps to reproduce:

1. As anonymous add product to the basket
2. Go to checkout
3. Provide address
4. Select shipping method and payment method
5. Proceed - you'll be redirected to the login screen (Side question: is this correct behavior?)
6. Go back to homepage
7. Go to checkout, proceed
8. Notice that system now remembers your address details, proceed

The following exception is thrown:

Cannot read property "identifier" from an array. Maybe you intended to write the property path as "[identifier]" instead. 

Solution:

It seems that \Ibexa\Bundle\Checkout\Controller\CheckoutStep\ShippingStepController is missing the following piece of code around line 104:

if (isset($options['context']['payment_method']['identifier'])) {
    $paymentMethodIdentifier = $options['context']['payment_method']['identifier'];

    $options['context']['payment_method'] = $this->paymentMethodService->getPaymentMethodByIdentifier($paymentMethodIdentifier);
}