Let’s assume an example where an obj has two children, both in a group, and one of them hidden. As hidden objs can’t be focused in group, the non-hidden one is the focused one.
When deleting this obj thus its two children, let’s assume the focused one is deleted first. From lv_obj_del(), lv_group_remove_obj() is called, where, in attempt to remove focus, lv_group_refocus() is called; but, as it can’t move focus to the only remaining object in group (the hidden one), focus won’t change. Subsequently, g->obj_focus = NULL; is executed, and the object in question is removed from the group’s LL.
In next step, the remaining hidden object is to be deleted, again lv_group_remove_obj() is called. There, third line is
if(g->obj_focus == NULL) return; /*Just to be sure (Not possible if there is at least one object in the group)*/
so it will return, without removing the obj from the group’s LL. However, the object is deleted by lv_obj_del() and the pointer to it becomes invalid.
In subsequent use of given group, this “dangling” pointer may cause crash.
I “fixed” the problem simply by removing the above line. I don’t know if that has or not any adverse consequences.
I am using v6.0. A quick glance into v7.0 shows that this piece of code is still there unchanged, so it may be affected, too.