[a11y] Treeview "Custom onAction": additional checkboxes can't be selected, and first selection opens an unannounced dialog

## Problem
In the "Custom onAction" example, once the first checkbox is selected, no further checkboxes can be selected. Activating that first checkbox also opens a pop-up with no advance warning to the user.

## Steps to Reproduce

1. Open the TreeView docs page and go to the "Custom onAction" example.
2. Select the first checkbox — note a pop-up opens.
3. Attempt to select any additional checkbox.
4.

## Expected Behavior
Any checkbox in the example can be selected or deselected independently. If activating a checkbox opens a dialog, that context change is communicated in advance via instructions or an accessible description.

## Actual Behavior
Only the first checkbox can be selected; subsequent ones do not respond. The first selection opens a pop-up with no prior indication, which is disorienting for keyboard and screen reader users.