message: Remove obsolete check
  The `StructureCatalogue::get` method will `throw` an exception rather
  than return `nullptr`.
diff: @@ -821,8 +821,6 @@ void MapViewState::placeStructure(Tile& tile)
  }
  Structure* structure = StructureCatalogue::get(mCurrentStructure);
  - if (!structure) { throw std::runtime_error("MapViewState::placeStructure(): NULL Structure returned from StructureCatalog."); }
  -
  NAS2D::Utility<StructureManager>::get().addStructure(*structure, tile);
  // FIXME: Ugly
  