Fix crash when closing panes very quickly (#17450)

#17358 introduced a bug where if you open/close panes very rapidly
Terminal will crash. This was because `_content` was being set to `null`
and then a `Close` event was being emitted, but several functions
attempt to access the pane's `_content` as part of the close routine.
For example, `TerminalTab` tries to update the `TaskbarProgress` every
time a pane is closed and as part of that update sequence it queries the
pane - which has `null` content now - for the taskbar progress,
resulting in a crash. This PR fixes that crash.

Refs #17358

(cherry picked from commit 5d46e31)
Service-Card-Id: 92776947
Service-Version: 1.21

src/cascadia/TerminalApp/Pane.cpp

@@ -1711,7 +1711,9 @@ void Pane::_SetupChildCloseHandlers()
 IPaneContent Pane::_takePaneContent()
 {
     _closeRequestedRevoker.revoke();
-    return std::move(_content);
+    // we cannot return std::move(_content) because we don't want _content to be null,
+    // since _content gets accessed even after Close is called
+    return _content;
 }
 
 // This method safely sets the content of the Pane. It'll ensure to revoke and
@@ -1721,15 +1723,14 @@ void Pane::_setPaneContent(IPaneContent content)
 {
     // The IPaneContent::Close() implementation may be buggy and raise the CloseRequested event again.
     // _takePaneContent() avoids this as it revokes the event handler.
-    if (const auto c = _takePaneContent())
+    if (_takePaneContent())
     {
-        c.Close();
+        _content.Close();
     }
 
-    _content = std::move(content);
-
-    if (_content)
+    if (content)
     {
+        _content = std::move(content);
         _closeRequestedRevoker = _content.CloseRequested(winrt::auto_revoke, [this](auto&&, auto&&) { Close(); });
     }
 }