Payment on site for cash and transfer method payment can cause Fatal error

[thorin87]

Dotpay extension version: 3.1.0
OC version: 3.0.3.2
Issue: When optional API credentials aren't set in extension settings and user choose cash or transfer method payments then request to extension/payment/dotpay_next/preparing ends with Fatal error
Reason:
$payment = $this->RegisterOrder->create($hiddenFields); (/catalog/controller/extension/payment/dotpay_next.php#L142) returns false.
Next line checks for $payment !== null, so condition is true despite $payment being false. As a result $this->model_extension_payment_dotpay_info->addInstruction ends with Fatal error