Fix NavigationObstacle nodes not registering to default navigation map

Fix NavigationObstacle nodes not registering to default navigation map.

scene/2d/navigation_obstacle_2d.cpp

@@ -139,7 +139,12 @@ void NavigationObstacle2D::set_navigation(Navigation2D *p_nav) {
 	}
 
 	navigation = p_nav;
-	Navigation2DServer::get_singleton()->agent_set_map(agent, navigation == nullptr ? RID() : navigation->get_rid());
+
+	if (navigation != nullptr) {
+		Navigation2DServer::get_singleton()->agent_set_map(agent, navigation->get_rid());
+	} else if (parent_node2d && parent_node2d->is_inside_tree()) {
+		Navigation2DServer::get_singleton()->agent_set_map(agent, parent_node2d->get_world_2d()->get_navigation_map());
+	}
 }
 
 void NavigationObstacle2D::set_navigation_node(Node *p_nav) {

scene/3d/navigation_obstacle.cpp

@@ -145,7 +145,12 @@ void NavigationObstacle::set_navigation(Navigation *p_nav) {
 	}
 
 	navigation = p_nav;
-	NavigationServer::get_singleton()->agent_set_map(agent, navigation == nullptr ? RID() : navigation->get_rid());
+
+	if (navigation != nullptr) {
+		NavigationServer::get_singleton()->agent_set_map(agent, navigation->get_rid());
+	} else if (parent_spatial && parent_spatial->is_inside_tree()) {
+		NavigationServer::get_singleton()->agent_set_map(agent, parent_spatial->get_world()->get_navigation_map());
+	}
 }
 
 void NavigationObstacle::set_navigation_node(Node *p_nav) {