get_node() starts looking from node that called it. So, if you calling it from script of HTTPRequest, you are looking for childs of HTTPRequest. And since there is no node named TextureRect as child of your HTTPRequest, it returns null. If you want to make this code work, you need to reparent TextureRect to HTTPRequest, so tree would look like
Spatial
- HTTPRequest
- TextureRect
Thanks @AlexTheREgent!
I wouldn’t like to change the tree. Only the path.
You’re right though that without slash it’s a child.