Render #pre_render callbacks must be methods of a class that implements

Get WSOD when trying to make a payment.

The error that follow this is:

Uncaught PHP Exception Drupal\\\\Core\\\\Security\\\\UntrustedCallbackException: "Render #pre_render callbacks must be methods of a class that implements \\\\Drupal\\\\Core\\\\Security\\\\TrustedCallbackInterface or be an anonymous function. The callback was Drupal\\\\commerce_partial_payments\\\\Plugin\\\\Commerce\\\\InlineForm\\\\PartialPaymentsGatewayForm::formatAmount. See https://www.drupal.org/node/2966725"