iemgui messages get sent for non-existent iemguis
There are several functions like
gobj_shouldvis which try to protect from sending unnecessary messages to the GUI.
However, there are still some cracks. For example,
slider_draw_config in g_slider.c will send a message to the GUI for a slider that is outside of the visible area on a graph-on-parent subcanvas. This means that the GUI must check for the object's existence before mutating it.