NullReferenceException thrown when box-selecting after deleting box-selected items

Steps to reproduce:

1. Open the "BiRP.zip" project
2. Open the "New Graph"
3. Create a new variable and instantiate it in the canvas
4. Box-select the variable in the canvas
5. Delete it
6. Box-select anything in the canvas
7. Observe the Console

Actual results: "NullReferenceException" is thrown

Expected results: Nothing is thrown

Reproducible with versions: 6000.5.1f1, 6000.6.0a4, 6000.6.0a7

Not reproducible with versions: 0.4.0-exp.2 (6000.3.18f1), 6000.4.12f1, 6000.6.0a3

Tested on (OS): Windows 11 Pro (25H2)

Notes:

• Works with multiple nodes and one alike
• Part of the error reads "NullReferenceException: Object reference not set to an instance of an object
  Unity.GraphToolkit.Editor.GraphElement.ShouldBeHighlighted"