Avoid reentrancy issues when dropping AppHost (#19296)

tl;dr: ~Apphost() may pump the message loop.
That's no bueno. See comments in the diff.

Additionally, this PR enables `_assertIsMainThread` in
release to trace down mysterious crashes in those builds.

src/cascadia/WindowsTerminal/WindowEmperor.cpp

@@ -549,6 +549,8 @@ void WindowEmperor::_dispatchSpecialKey(const MSG& msg) const
 
 void WindowEmperor::_dispatchCommandline(winrt::TerminalApp::CommandlineArgs args)
 {
+    _assertIsMainThread();
+
     const auto exitCode = args.ExitCode();
 
     if (const auto msg = args.ExitMessage(); !msg.empty())
@@ -686,6 +688,8 @@ safe_void_coroutine WindowEmperor::_dispatchCommandlineCurrentDesktop(winrt::Ter
 
 bool WindowEmperor::_summonWindow(const SummonWindowSelectionArgs& args) const
 {
+    _assertIsMainThread();
+
     AppHost* window = nullptr;
 
     if (args.WindowID)
@@ -726,6 +730,8 @@ bool WindowEmperor::_summonWindow(const SummonWindowSelectionArgs& args) const
 
 void WindowEmperor::_summonAllWindows() const
 {
+    _assertIsMainThread();
+
     TerminalApp::SummonWindowBehavior args;
     args.ToggleVisibility(false);
 
@@ -863,6 +869,9 @@ LRESULT WindowEmperor::_messageHandler(HWND window, UINT const message, WPARAM c
                 // Did the window counter get out of sync? It shouldn't.
                 assert(_windowCount == gsl::narrow_cast<int32_t>(_windows.size()));
 
+                // !!! NOTE !!!
+                // At least theoretically the lParam pointer may be invalid.
+                // We should only access it if we find it in our _windows array.
                 const auto host = reinterpret_cast<AppHost*>(lParam);
                 auto it = _windows.begin();
                 const auto end = _windows.end();
@@ -871,7 +880,15 @@ LRESULT WindowEmperor::_messageHandler(HWND window, UINT const message, WPARAM c
                 {
                     if (host == it->get())
                     {
-                        host->Close();
+                        // NOTE: The AppHost destructor is highly non-trivial.
+                        //
+                        // It _may_ call into XAML, which _may_ pump the message loop, which would then recursively
+                        // re-enter this function, which _may_ then handle another WM_CLOSE_TERMINAL_WINDOW,
+                        // which would change the _windows array, and invalidate our iterator and crash.
+                        //
+                        // We can prevent this by deferring destruction until after the erase() call.
+                        const auto strong = *it;
+                        strong->Close();
                         _windows.erase(it);
                         break;
                     }

src/cascadia/WindowsTerminal/WindowEmperor.h

@@ -84,14 +84,14 @@ private:
     int32_t _windowCount = 0;
     int32_t _messageBoxCount = 0;
 
-#ifdef NDEBUG
+#if 0 // #ifdef NDEBUG
     static constexpr void _assertIsMainThread() noexcept
     {
     }
 #else
     void _assertIsMainThread() const noexcept
     {
-        assert(_mainThreadId == GetCurrentThreadId());
+        WI_ASSERT_MSG(_mainThreadId == GetCurrentThreadId(), "This part of WindowEmperor must be accessed from the UI thread");
     }
     DWORD _mainThreadId = GetCurrentThreadId();
 #endif