"NullReferenceException" errors are spammed when adding a second count-type query block in Systems window or typing in search while row item is selected

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 + icon in the search to add query block
5. Select "Component Count" block
6. Select any filtered option
7. Click + icon again and add "Dependency Count" block
8. Observe the Console

Actual results: "NullReferenceException: Object reference not set to an instance of an object Unity.Entities.Editor.SystemTreeViewItemData.get_HasChildren" errors are spammed

Expected results: No errors

Reproducible with versions: 6000.6.0a6

Not reproducible with versions: 6000.5.0b10, 6000.6.0a5

Tested on (OS): Ubuntu 24.04, Windows 11

Notes:

• Errors keep spamming until Systems window is closed
• Also seems to reproduce when just searching in the window while another row element has been selected after 6th step
• Reproduces with other blocks in 7th step too ("Entity Count", "ChildCount")