Fix single-tab window tear-off crash (#17251)

## Validation Steps Performed
- Opened multi-tab terminal window with Narrator. Narrator can read
characters from the tabs.
- Started a drag and drop (tear-off) of a tab, and it didn't crash. This
was repeated multiple times.

(cherry picked from commit 30ef1f4)
Service-Card-Id: 92546868
Service-Version: 1.20

src/cascadia/TerminalControl/TermControl.cpp

@@ -940,8 +940,11 @@ namespace winrt::Microsoft::Terminal::Control::implementation
         // MSFT 33353327: We're purposefully not using _initializedTerminal to ensure we're fully initialized.
         // Doing so makes us return nullptr when XAML requests an automation peer.
         // Instead, we need to give XAML an automation peer, then fix it later.
-        if (!_IsClosing())
+        if (!_IsClosing() && !_detached)
         {
+            // It's unexpected that interactivity is null even when we're not closing or in detached state.
+            THROW_HR_IF_NULL(E_UNEXPECTED, _interactivity);
+
             // create a custom automation peer with this code pattern:
             // (https://docs.microsoft.com/en-us/windows/uwp/design/accessibility/custom-automation-peers)
             if (const auto& interactivityAutoPeer{ _interactivity.OnCreateAutomationPeer() })