subsurface: handle NULL parent in get_root_surface

include/wlr/types/wlr_surface.h

@@ -220,7 +220,8 @@ struct wlr_subsurface *wlr_subsurface_create(struct wlr_surface *surface,
 		struct wl_list *resource_list);
 
 /**
- * Get the root of the subsurface tree for this surface.
+ * Get the root of the subsurface tree for this surface. Can return NULL if
+ * a surface in the tree has been destroyed.
  */
 struct wlr_surface *wlr_surface_get_root_surface(struct wlr_surface *surface);
 

types/wlr_surface.c

@@ -1061,6 +1061,9 @@ struct wlr_surface *wlr_surface_get_root_surface(struct wlr_surface *surface) {
 		if (subsurface == NULL) {
 			break;
 		}
+		if (subsurface->parent == NULL) {
+			return NULL;
+		}
 		surface = subsurface->parent;
 	}
 	return surface;