Merge branch 'master' into jgrpp-nrt

Merge NRT feature

# Conflicts:
#	docs/landscape.html
#	docs/landscape_grid.html
#	src/bridge_map.h
#	src/build_vehicle_gui.cpp
#	src/company_base.h
#	src/company_cmd.cpp
#	src/misc_gui.cpp
#	src/newgrf.cpp
#	src/newgrf_engine.cpp
#	src/pathfinder/follow_track.hpp
#	src/pathfinder/npf/npf.cpp
#	src/road_cmd.cpp
#	src/road_func.h
#	src/road_gui.cpp
#	src/road_map.h
#	src/road_type.h
#	src/roadveh_cmd.cpp
#	src/saveload/afterload.cpp
#	src/saveload/company_sl.cpp
#	src/script/api/script_bridge.cpp
#	src/table/newgrf_debug_data.h
#	src/tile_cmd.h
#	src/town_cmd.cpp
#	src/tunnel_map.h
#	src/tunnelbridge_cmd.cpp

src/rail_gui.cpp

@@ -2235,5 +2235,11 @@ DropDownList GetRailTypeDropDownList(bool for_replacement, bool all_option)
 		item->SetParam(1, rti->max_speed);
 		list.emplace_back(item);
 	}
+
+	if (list.size() == 0) {
+		/* Empty dropdowns are not allowed */
+		list.emplace_back(new DropDownListStringItem(STR_NONE, INVALID_RAILTYPE, true));
+	}
+
 	return list;
 }