[Ignition] PostRender event not called

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Hi all, I have a problem when integrating a camera plugin that I designed in ROS. If I use the plugin in a world created with an SDF it correctly works, but if I attach it to a camera declared in a URDF to be used in ROS, the PostRender event (ignition::gazebo::events::PostRender) is never called and I cannot understand why. The plugin is correctly loaded, the Configure and PostUpdate callbacks are correctly called.

Any idea?

Thank you Walter