Add a better error rather than giving an unhelpful Null PMC Access for a possible failure mode in P6opaque.
Add a better error rather than giving an unhelpful Null PMC Access for a possible failure mode in P6opaque.

cygx/nqp,cygx/nqp,cygx/nqp,cygx/nqp,cygx/nqp,cygx/nqp,cygx/nqp,cygx/nqp