Controller::curr() should return null instead of throwing an exception

[GuySartorelli]

Controller::curr() currently throws an exception if there's no current controller. This means you have to use Controller::has_curr() any time you want to call curr() - when a null return value would do just as well.

Related issues

• Reduce static coupling to Controller::has_curr() and Controller::curr() #4484

Acceptance Criteria

• Controller::curr() returns null if there's no current controller (instead of throwing an exception).