[Bug]: TypeError with Camera "follow_entity" method

[EZ-Blue]

Bug Description

Very small bug:

I'm encountering a [ERROR] TypeError: 'NoneType' object does not support item assignment when trying to use the follow_entity() method of the Camera class.

This happens when the argument fix_orientation is passed with the value True AND no argument is provided for smoothing.

In camera.py in the update_following method it sets camera_lookat = None if fix_orientation == True.

If smoothing is also None, then it enters an else statement that attempts to directly set the camera_pos[:] and camera_lookat[:] to the entity_pos, which raises the NoneType item assignment error since camera_lookat is still None. This is Lines 349-350 in camera.py.

This doesn't happen when a value is provided for the smoothing argument since camera_lookat just isn't updated, so I'm not sure if the fix_orientation method was only intended to be used along with some smoothing value.

I've attached a minimal code to reproduce the error.

Steps to Reproduce

If possible, provide a script triggering the bug, e.g.

# code snippet triggering the bug
import genesis as gs

def main():
    
    gs.init()
    
    device = gs.device

    scene = gs.Scene(
        sim_options=gs.options.SimOptions(dt=0.01, substeps=2),
        viewer_options=gs.options.ViewerOptions(
            camera_pos=(3.5, 0.0, 2.5),
            camera_lookat=(0.0, 0.0, 0.5),
            camera_fov=40,
            max_FPS=60,
        ),
        vis_options=gs.options.VisOptions(
            rendered_envs_idx=list(range(1)),
        ),
        profiling_options=gs.options.ProfilingOptions(show_FPS=False),
        show_viewer=True,
    )

    plane = scene.add_entity(morph=gs.morphs.Plane())

    # Add camera
    cam = scene.add_camera(
            res=(640, 480),
            pos=(3.5, 0.0, 2.5),
            lookat=(0, 0, 0.7),
            fov=40,
            GUI=True,
        )
    
    franka = scene.add_entity(
        gs.morphs.MJCF(
            file="xml/franka_emika_panda/panda.xml",
        ),
    )

    scene.build()

    cam.follow_entity(franka, fix_orientation=True)

    for i in range(1000):
        scene.step()

if __name__ == "__main__":
    main()

Expected Behavior

Camera should update to look at the specified entity without raising error.

Screenshots/Videos

No response

Relevant log output

Environment

• OS: Ubuntu 22.04
• GPU: RTX 5080
• GPU-driver version: 570.195.03
• CUDA / CUDA-toolkit version: 12.8

Release version or Commit ID

Tested on v0.3.6, v0.3.5 and v0.3.3.

Additional Context

No response