[UI] Attendance window is not closed upon a new `getattg` command

[anselmlong]

Overview:

Running getattg with any tutorial group multiple times just creates multiple attendance windows.

Environment:

Processor: 13th Gen Intel(R) Core(TM) i5-13500H, 2600 Mhz, 12 Core(s), 16 Logical Processor(s) Operating System: Microsoft Windows 11 Home Version 10.0.22631 Build 22631 App Version: v1.6

Steps to Reproduce:

1. Add one student to tutorial group X
2. Run getattg tg/X
3. Repeat step 2
4. 2 windows open

Expected Result:

The program should check if a window is currently open, and bring it up instead. Either that, or it closes the current window open.

Actual Result:

No checks, opens multiple windows.

Screenshots/Logs (if applicable):

Frequency:

Always