Safely check if teacup_next_responder weakref is still alive

We've been getting some WeakRef::RefError crash reports.  I've come up with a solution.  It is not the most elegant one, but does solve the problem.

WeakRefs are nice, but you can't do anything with them beside check if they are alive if they are no more.  You cannot even check if they exist.  This solves that by setting a flag to know if we have one.

I could not find anywhere that nils it out, so I believe this is sufficient.