SimpleActionState crashes during request_preempt

[jcsantamaria]

In a SMACH state machine that includes an instance of SimpleActionState, if that state is active and one issues a request_preempt(), the program crashes.
The main reason is that the SimpleActionState.request_preempt() method attempts to call self._action_client.cancel_goal() as seen here (link), but the ActionClient class does not have such method (see reference). The cancel_goal() method does exists but it is on the ClientGoalHandle class.
I have a fix in place. I could share it as a contribution, but I could not find the contributions rules/instructions in this repo. Please advise.