Windows: every start ended itself after 3 s (1.3.0, 1.3.1)

Win32Window::Create calls Destroy() before it makes the window, which
ran FlutterWindow::OnDestroy and armed the 3 s exit timer at launch: the
window showed, then the process ended with code 0, no crash, no close
lines. The timer is only armed with an engine to tear down. Found and
fixed by Fun Maker's agent on Windows (a debugger caught the exit on the
timer's thread).

The smoke runs missed it (they closed at ~4 s anyway): the app now stays
up 10 s, and the scripts require 'closing' and 'quitting' from its own
close.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

apps/desktop/windows/runner/flutter_window.cpp

@@ -80,7 +80,11 @@ bool FlutterWindow::OnCreate() {
 
 void FlutterWindow::OnDestroy() {
   // The engine and the plugins go now; the app released its players first.
-  ExitAfter(3000);
+  // Win32Window::Create runs this too, before there is an engine: only a
+  // real teardown may end the process (else every start dies at 3 s).
+  if (flutter_controller_) {
+    ExitAfter(3000);
+  }
   window_channel_ = nullptr;
   if (flutter_controller_) {
     flutter_controller_ = nullptr;