GdkSurface::set_startup_id() is NULL on Win32 and would cause a segfault if called.
While the documentation of the main caller of set_startup_id(), gtk_window_set_startup_id(), mentions that it's not implemented on Windows it can still be automatically called via Glade and simply doing nothing on Win32 is going to be less disruptive than a segfault.
See https://github.com/gtk-rs/gtk/issues/811 and !825 (merged) for the master MR.