Explicit null dereferenced

A null pointer dereference will occur. Dereference of an explicit null value
/src/mongo/db/pipeline/document_source_union_with.cpp:158: FORWARD_NULL 177476 "this->_sharedState" is initialized empty.
/src/mongo/db/pipeline/document_source_union_with.cpp:197: FORWARD_NULL 177476 Attempting to access the managed object of an empty smart pointer "this->_sharedState".