COMP: Fix return error in MeshIO::write
System: gcc (Ubuntu 5.4.0-6ubuntu1~16.04.4) 5.4.0 20160609
error: converting to ‘bool’ from ‘std::nullptr_t’ requires direct-initialization
System: gcc (Ubuntu 5.4.0-6ubuntu1~16.04.4) 5.4.0 20160609
error: converting to ‘bool’ from ‘std::nullptr_t’ requires direct-initialization