fix: check whether lastFocusedList is valid when assigned

[oxcened]

Fixes #234791

Steps to Reproduce

1. Open a terminal.
2. Click on the terminal name button.
3. Choose "Change color..." and select any color in the modal.
4. Click on the terminal name button again.
5. Try "Rename...", "Split terminal", or "Kill terminal."
6. Notice the buttons don't work.

Issue

The problem happens when you pick a color from the modal. When the color is selected, the modal briefly grabs focus, which triggers an onDidFocus event. This event updates _lastFocusedWidget in listService, which is then mistakenly used in the function getSelectedInstances. The function assumes that lastFocusedList is always a TerminalTabList, which isn't always true.

Fix

The fix ensures listService.lastFocusedList is only used when it's a TerminalTabList, not when other widgets like the QuickInputTree are focused.

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Let me know if any adjustments are needed!