tracking_video_renderer invalid ID issue

[thoschuller]

In the current main branch, when trying to record a video using the tracking_video_renderer, it raises an error "mjv_updateCamera: track body id is outside valid range" after an INFO log of "Tracking core body ID: -1". This happens both without giving a name to track and when giving the name "core". Last known working commit for me was this.

Issue happens on both windows and linux with python 3.12.9.

Easily reproducable by running examples/z_ec_course/A3_template_jack.py after changing line 252 to mode="tracking".