Dereference of potentially null field

If the field is actually null, then a null pointer dereference will occur. Field, which is statistically checked against null, is dereferenced.
/src/mongo/db/pipeline/document_source_facet.cpp:222: NULL_FIELD 176941 Reading field "execPipeline", which is expected to possibly be "nullptr" in "this->_facets[facetId].execPipeline" (checked 9 out of 10 times).
/src/mongo/db/pipeline/document_source_facet.cpp:222: NULL_FIELD 176941 Attempting to access the managed object of an empty smart pointer "this->_facets[facetId].execPipeline".