fix crash on fullscreen toggle

sway/tree/view.c

@@ -163,8 +163,9 @@ void view_set_fullscreen_raw(struct sway_view *view, bool fullscreen) {
 		struct sway_container *focus, *focus_ws;
 		wl_list_for_each(seat, &input_manager->seats, link) {
 			focus = seat_get_focus(seat);
+			if (focus) {
 				focus_ws = focus;
-			if (focus_ws->type != C_WORKSPACE) {
+				if (focus && focus_ws->type != C_WORKSPACE) {
 					focus_ws = container_parent(focus_ws, C_WORKSPACE);
 				}
 				seat_set_focus(seat, view->swayc);
@@ -172,6 +173,7 @@ void view_set_fullscreen_raw(struct sway_view *view, bool fullscreen) {
 					seat_set_focus(seat, focus);
 				}
 			}
+		}
 	} else {
 		workspace->sway_workspace->fullscreen = NULL;
 		view->swayc->width = view->swayc->saved_width;