How to set the material attribute in particles2d when node is ready?

origami

I check this link https://godotengine.org/qa/38691/change-property-of-particlesmaterial-with-code

and get the particles.material.set("angular_velocity", 2.5).

I want to put it in the ready function. But I get error message:

Attempt to call function 'set' in base 'null instance' on a null instance. I also put it in the tree_entered signal responding function not work too. So how to set it at start.

One more question, what is the difference between ready and tree_entered?

tree_entered is earlier than ready, right?

origami

This is worked process_material.set("angular_velocity", 2.5) . Sorry if it is disturbing.