Revert "fix #3526 (falling trees cause segfault)"

This reverts commit a3ff0943cdbeb31cdb3ab3c1721eb1af2451e1ad.
fix #3526 without being paranoid (only falling trees were causing problems) or breaking featureLOS rules again

furthermore the added assert was wrong:
1) it is NOT an error for a featureID to map to NULL, same as with unitID's
2) non-falling trees get removed via TreeDrawer::DeleteTree --> their IDs can NOT map to NULL