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/main_gui.cpp

@@ -157,7 +157,7 @@ void FixTitleGameZoom(int zoom_adjust)
 {
 	if (_game_mode != GM_MENU) return;
 
-	Viewport *vp = FindWindowByClass(WC_MAIN_WINDOW)->viewport;
+	Viewport *vp = GetMainWindow()->viewport;
 
 	/* Adjust the zoom in/out.
 	 * Can't simply add, since operator+ is not defined on the ZoomLevel type. */