Check for multi backend before using it

backend/multi/backend.c

@@ -66,6 +66,10 @@ struct wlr_backend *wlr_multi_backend_create(struct wlr_session *session,
 	return backend;
 }
 
+bool wlr_backend_is_multi(struct wlr_backend *b) {
+	return b->impl == &backend_impl;
+}
+
 static void input_add_reemit(struct wl_listener *listener, void *data) {
 	struct subbackend_state *state = wl_container_of(listener,
 			state, input_add);

examples/shared.c

@@ -48,10 +48,12 @@ static void keyboard_key_notify(struct wl_listener *listener, void *data) {
 		} else if (key_state == WLR_KEY_PRESSED &&
 				sym >= XKB_KEY_XF86Switch_VT_1 &&
 				sym <= XKB_KEY_XF86Switch_VT_12) {
-			struct wlr_session *session =
-				wlr_multi_get_session(kbstate->compositor->backend);
-			if (session) {
-				wlr_session_change_vt(session, sym - XKB_KEY_XF86Switch_VT_1 + 1);
+			if (wlr_backend_is_multi(kbstate->compositor->backend)) {
+				struct wlr_session *session =
+					wlr_multi_get_session(kbstate->compositor->backend);
+				if (session) {
+					wlr_session_change_vt(session, sym - XKB_KEY_XF86Switch_VT_1 + 1);
+				}
 			}
 		}
 	}

include/wlr/backend/multi.h

@@ -10,6 +10,8 @@ struct wlr_backend *wlr_multi_backend_create(struct wlr_session *session,
 void wlr_multi_backend_add(struct wlr_backend *multi,
 		struct wlr_backend *backend);
 
+bool wlr_backend_is_multi(struct wlr_backend *backend);
+
 struct wlr_session *wlr_multi_get_session(struct wlr_backend *base);
 
 #endif