Unable to load membership renewal form on back end

This appears to be a regression caused by !7 (merged). Admin users are no longer able to load the membership renewal form. When they try to do so they get an error and a blank form.

Looking at the error log I can see that this is related to the change in !7 (merged):

Error: Cannot access protected property CRM_Member_Form_MembershipRenewal::$_params in CRM_Core_Payment_Smartdebit->buildForm() (line 164 of /var/www/html/sites/all/civicrm_extensions/org.civicrm.smartdebit/CRM/Core/Payment/Smartdebit.php).