data-control-v1: handle NULL sources in set_selection

types/wlr_data_control_v1.c

@@ -177,11 +177,15 @@ static void control_handle_set_selection(struct wl_client *client,
 		struct wl_resource *source_resource) {
 	struct wlr_data_control_device_v1 *device =
 		control_from_resource(control_resource);
-	struct client_data_source *source = source_from_resource(source_resource);
 	if (device == NULL) {
 		return;
 	}
 
+	struct client_data_source *source = NULL;
+	if (source_resource != NULL) {
+		source = source_from_resource(source_resource);
+	}
+
 	struct wlr_data_source *wlr_source = source ? &source->source : NULL;
 	wlr_seat_request_set_selection(device->seat, wlr_source,
 		wl_display_next_serial(device->seat->display));