Fix a crash on exit with the command palette open (#13778)

Fixes MSFT:38775539
Might also fix MSFT:38614563

Looking at this code should be pretty clear what's going on. On exit, the XAML root is already nulled out. But here, we're just yolo'ing and assuming it exists (why wouldn't it). So yea. This is like weirdly high percent of crashes internally, but that's not from real users. Real users, I suspect hit this as like .3% of our crashes. Not zero, but _low_.

* [x] tested manually

<hr>

May also be related to...
* MSFT:40602905
* MSFT:40602904
* MSFT:40412800
* MSFT:35213459 <---has links

(cherry picked from commit 64bcc0b)
Service-Card-Id: 85486788
Service-Version: 1.15

src/cascadia/TerminalApp/CommandPalette.cpp

@@ -474,7 +474,13 @@ namespace winrt::TerminalApp::implementation
             return;
         }
 
-        auto focusedElementOrAncestor = Input::FocusManager::GetFocusedElement(this->XamlRoot()).try_as<DependencyObject>();
+        auto root = this->XamlRoot();
+        if (!root)
+        {
+            return;
+        }
+
+        auto focusedElementOrAncestor = Input::FocusManager::GetFocusedElement(root).try_as<DependencyObject>();
         while (focusedElementOrAncestor)
         {
             if (focusedElementOrAncestor == *this)