Fix wrong CommandPallete access (#17069)

Closes: #17032

We were wrongly calling the Ctor of CommandPalette which led to the
creation of an uninitialized winrt command palette object, and then
OnCreateAutomationPeer() was called on that. This seems to be the cause
of #17032.

## Validation Steps Performed
- Open WT.
- Try to tear off a tab out of the tab headers view.
- WT doesn't crash.

src/cascadia/TerminalApp/TabManagement.cpp

@@ -881,7 +881,10 @@ namespace winrt::TerminalApp::implementation
             }
         }
 
-        CommandPalette().Visibility(Visibility::Collapsed);
+        if (const auto p = CommandPaletteElement())
+        {
+            p.Visibility(Visibility::Collapsed);
+        }
         _UpdateTabView();
     }