PanelRenderer UIReloadCallback accumulates code-created content

Steps to reproduce:

1. Create a PanelRenderer component
2. Create a MonoBehaviour that registers UIReloadCallback  in OnEnable
3. In the callback, add more VisualElements in the root
4. Hit play
5. Disable and re-enable the MonoBehaviour component on and off

Actual results: The VisualElements created in code accumulates

Expected results: No accumulation

Reproducible with versions: 

Not reproducible with versions: 

Can’t test with versions: 

Tested on (OS): 

Notes:

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