fix(windows): release FFmpeg memory and capture native crashes (#1668)

ui/flutter/windows/runner/flutter_window.cpp

@@ -63,7 +63,9 @@ FlutterWindow::MessageHandler(HWND hwnd, UINT const message,
 
   switch (message) {
     case WM_FONTCHANGE:
-      flutter_controller_->engine()->ReloadSystemFonts();
+      if (flutter_controller_ && flutter_controller_->engine()) {
+        flutter_controller_->engine()->ReloadSystemFonts();
+      }
       break;
   }