Hey,
this is happening because WC()->payment_gateways->get_available_payment_gateways() is not ment to be used the way the plugin is using it. Some payment gateways implement custom is_available() method and, for example, expect that WC()->session or WC()->cart exists in the time of calling, otherwise they will crash with “PHP Fatal error: Call to a member function XXX() on null” leading to the problem the original poster is experiencing.
This is the same reason why the WooCommerce in-built COD payment gateway is not listed on plugins settings page – it_available() checks if cart needs shipping or if the page requested is checkout page – which both is, of course, false, because we’re on the admin plugin settings page.
To resolve the issue I suggest you should replace WC()->payment_gateways->get_available_payment_gateways() with WC()->payment_gateways->payment_gateways() and filter out the gateways that are not enabled.
