use nullptr

Common/GmshMessage.cpp

Common/Main.cpp

Common/OS.cpp

Common/OctreeInternals.cpp

Fltk/FlGui.cpp

Fltk/fileDialogs.h

Fltk/touchBar.mm

Geo/GModelCreateTopologyFromMesh.cpp

Geo/GModelIO_GEO.cpp

Geo/GModelIO_MSH2.cpp

Geo/MEdge.cpp

Geo/closestVertex.cpp

Graphics/PixelBuffer.h

Graphics/gl2ps.cpp

Graphics/gl2ps.h

Mesh/Field.cpp

Mesh/meshGRegionBoundaryRecovery.cpp

Mesh/meshGRegionMMG.cpp

Parser/Gmsh.tab.cpp

Parser/Gmsh.y