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/window_func.h

@@ -16,6 +16,7 @@
 
 Window *FindWindowById(WindowClass cls, WindowNumber number);
 Window *FindWindowByClass(WindowClass cls);
+Window *GetMainWindow();
 void ChangeWindowOwner(Owner old_owner, Owner new_owner);
 
 void ResizeWindow(Window *w, int x, int y, bool clamp_to_screen = true);