Added handling to deal with if the lambda function passed in is a nullptr type.
Added handling to deal with if the lambda function passed in is a nullptr type.

alexrj/Slic3r,curieos/Slic3r,lordofhyphens/Slic3r,curieos/Slic3r,lordofhyphens/Slic3r,alexrj/Slic3r,alexrj/Slic3r,curieos/Slic3r,curieos/Slic3r,alexrj/Slic3r,pieis2pi/Slic3r,lordofhyphens/Slic3r,curieos/Slic3r,lordofhyphens/Slic3r,pieis2pi/Slic3r,pieis2pi/Slic3r,curieos/Slic3r,lordofhyphens/Slic3r,pieis2pi/Slic3r,alexrj/Slic3r,alexrj/Slic3r,pieis2pi/Slic3r,pieis2pi/Slic3r,lordofhyphens/Slic3r