getSignResponse may return null in Helper/Cart and result in nonexistent member function error

[jsamhall]

Currently on version 2.26.0 but issue appears present upstream as well

In Helper/Cart, the private function getSignResponse may return null in the event of an issue with API:

That possibility is ignored in calling code in the method getHints:

When this occurs, a CRITICAL level system log is issued:

 main.CRITICAL: Error: Call to a member function getResponse() on null in /.../vendor/boltpay/bolt-magento2/Helper/Cart.php:1084