"NullReferenceException" are spammed and Systems window flickers when enabling "Show Hidden Entities" from the Systems window's contextual Preferences menu

Steps to reproduce:

1. Create a new 3D URP project
2. Install the Entities package (com.unity.entities)
3. Go to Window > Entities > Systems
4. Click on the lower three dots in the window
5. Select "Entities Preferences"
6. In the Preferences, enable "Show Hidden Entities" property
7. Observe the Console and Systems window

Actual results: "NullReferenceException: Object reference not set to an instance of an object" errors are thrown and Systems window entries are constantly flickering/refreshing

Expected results: No errors or flickering is observed

Reproducible with versions: 6000.5.0a8, 6000.5.0b10, 6000.6.0a6

Can’t test with versions: 6000.4.9f1 ("Show Hidden Entities" option does not exist)

Tested on (OS): Ubuntu 24.04, Windows 11

Notes:

• Errors are spammed until the Systems window is closed
• Does not reproduce when "Show Hidden Entities" is enabled via Edit > Preferences > Entities