hc:PinBox FocusManager.FocusedElement not work

[GF-Huang]

Describe the bug

To Reproduce
Steps to reproduce the behavior:

1. Go to '...'
2. Click on '....'
3. Scroll down to '....'
4. See error

Expected behavior
When the UnlockView opened, the PinBox auto get focus without mouse to click the first box of it.

Screenshots
If applicable, add screenshots to help explain your problem.

Environment (please complete the following information):

• .net: 5
• IDE VS2019
• Version HandyControls 3.3.8.1

Additional context
Add any other context about the problem here.