Codechange: introduce GetMainWindow() to properly account for nullptr checks

Some nullptr checks have been removed as they were not triggered with nullptr
with the null video driver and in dedicated server mode.

src/genworld.cpp

@@ -321,9 +321,7 @@ void GenerateWorld(GenWorldMode mode, uint size_x, uint size_y, bool reset_setti
 	ShowGenerateWorldProgress();
 
 	/* Centre the view on the map */
-	if (FindWindowById(WC_MAIN_WINDOW, 0) != nullptr) {
-		ScrollMainWindowToTile(TileXY(MapSizeX() / 2, MapSizeY() / 2), true);
-	}
+	ScrollMainWindowToTile(TileXY(MapSizeX() / 2, MapSizeY() / 2), true);
 
 	_GenerateWorld();
 }