Update tutorial to avoid moving the MTC task

[stephanie-eng]

Description

Running the existing MTC tutorial code will segfault with the latest version of MoveIt Task Constructor. Workaround is to avoid moving/copying the MTC task.

Checklist

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[henrygerardmoore]

Cloned and ran and it works as expected now! Didn't encounter any problems

[rhaschke]

Obviously, it would be better to fix MTC. Is there an issue for the underlying problem? Could you pinpoint the problem?
I think this worked with MoveIt1.

[JafarAbdi]

Obviously, it would be better to fix MTC. Is there an issue for the underlying problem? Could you pinpoint the problem? I think this worked with MoveIt1.

@rhaschke I was debugging this issue yesterday and came up with a minimal reproducible example, I'll open a draft PR later today

TEST(Task, taskMoveContructor) {
	rclcpp::init(0, nullptr);
	auto createTask = [] {
		moveit::core::RobotModelConstPtr robot_model = getModel();
		Task t("first");
		t.setRobotModel(robot_model);
		resetMockupIds();
		auto ref = new stages::FixedState("fixed");
		auto scene = std::make_shared<planning_scene::PlanningScene>(t.getRobotModel());
		ref->setState(scene);

		t.add(Stage::pointer(ref));
		t.add(std::make_unique<ConnectMockup>());
		t.add(std::make_unique<MonitoringGeneratorMockup>(ref));
		return t;
	};

	// Doesn't segfault
	/* Task t = createTask(); */
	// Segfaults when introspection is enabled
	Task t;
	t = createTask();

	try {
		t.init();
		EXPECT_TRUE(t.plan(1));
	} catch (const InitStageException& e) {
		ADD_FAILURE() << "InitStageException:" << std::endl << e << t;
	}
}

[stephanie-eng]

I agree that we should fix MTC as well, but I also think this workaround is a better usage example for the tutorials. It's a bit cleaner to read, and there's really no need to move the task around when we can just pass a reference to it.

[rhaschke]

It's a bit cleaner to read, and there's really no need to move the task around when we can just pass a reference to it.

I don't really agree. I think it is quite natural to have a factory function for task creation instead of "filling" a provided task instance.

[tylerjw]

Generally, I think it is not advisable to use mutable reference parameters. I agree that the better option is to fix MTC than to require/encourage people to create mutable references.

[stephanie-eng]

OK, then we can leave the tutorial as-is - I'll close this PR once MTC is fixed.