Possible NPE when custom cloner is badly written

When user creates his own cloner, planner does not check if cloner works properly. Cloned solution could for example contain score as null and throws NPE during next solution comparison.

Actual results:
when cloner is badly written, planner throws NPE (but there are many possibilities what can go wrong)

Expected results:
At least FULL_ASSERT mode should detect bad cloner, but I think that checking cloner before every solving is not performance killer, so planner could check it always and throws some exception if needed.