check for null slabs
check for null slabs

Makes rendering work when there is no object added
to the viewer.

CyrusBiotechnology/pv,Traksewt/pv,Traksewt/pv,biasmv/pv,biasmv/pv,CyrusBiotechnology/pv,Traksewt/pv,biasmv/pv,CyrusBiotechnology/pv,CyrusBiotechnology/pv,yuhangwang/pv,yuhangwang/pv,Traksewt/pv,yuhangwang/pv,biasmv/pv,yuhangwang/pv