Remove the window thread from the list of threads before nulling the AppHost (#15231)

See
https://github.com/microsoft/terminal/issues/14957#issuecomment-1520522722.

I think there's a race here that lets the WindowEmperor muck around with
the window after it's done, but before we remove it from our list of
threads.

This _should_ remove the thread from the list, _then_ null out the
AppHost, then flush the XAML queue, preventing the A/V.

Closes MSFT:43995981

src/cascadia/WindowsTerminal/WindowEmperor.cpp

@@ -140,7 +140,7 @@ void WindowEmperor::_createNewWindowThread(const Remoting::WindowRequestedArgs&
     std::thread t([weakThis, window]() {
         try
         {
-            const auto cleanup = wil::scope_exit([&]() {
+            auto cleanup = wil::scope_exit([&]() {
                 if (auto self{ weakThis.lock() })
                 {
                     self->_windowExitedHandler(window->Peasant().GetID());
@@ -155,6 +155,16 @@ void WindowEmperor::_createNewWindowThread(const Remoting::WindowRequestedArgs&
             }
 
             window->RunMessagePump();
+
+            // Manually trigger the cleanup callback. This will ensure that we
+            // remove the window from our list of windows, before we release the
+            // AppHost (and subsequently, the host's Logic() member that we use
+            // elsewhere).
+            cleanup.reset();
+
+            // Now that we no longer care about this thread's window, let it
+            // release it's app host and flush the rest of the XAML queue.
+            window->RundownForExit();
         }
         CATCH_LOG()
     });

src/cascadia/WindowsTerminal/WindowThread.cpp

@@ -37,7 +37,11 @@ int WindowThread::RunMessagePump()
     // Enter the main window loop.
     const auto exitCode = _messagePump();
     // Here, the main window loop has exited.
+    return exitCode;
+}
 
+void WindowThread::RundownForExit()
+{
     _host = nullptr;
     // !! LOAD BEARING !!
     //
@@ -54,8 +58,6 @@ int WindowThread::RunMessagePump()
             ::DispatchMessageW(&msg);
         }
     }
-
-    return exitCode;
 }
 
 winrt::TerminalApp::TerminalWindow WindowThread::Logic()

src/cascadia/WindowsTerminal/WindowThread.h

@@ -15,6 +15,7 @@ public:
     winrt::TerminalApp::TerminalWindow Logic();
     void CreateHost();
     int RunMessagePump();
+    void RundownForExit();
 
     winrt::Microsoft::Terminal::Remoting::Peasant Peasant();