throw a C# exception when window.instance is null instead of crashing down the app

com.unity.uiwidgets/Runtime/ui2/window.cs

 
         public static Window instance {
             get {
-                GCHandle gcHandle = (GCHandle) Window_instance();
+                IntPtr ptr = Window_instance();
+                if (ptr == IntPtr.Zero) {
+                    D.assert(false, () => "AssertionError: Window.instance is null. Please enclose your code with window scope (detailed examples can be found in the README file)");
+                    return null;
+                }
+                
+                GCHandle gcHandle = (GCHandle) ptr;
                 return (Window) gcHandle.Target;
             }
         }

engine/src/lib/ui/ui_mono_state.cc

   return static_cast<UIMonoState*>(MonoState::Current());
 }
 
+bool UIMonoState::EnsureCurrentIsolate() {
+  return MonoState::EnsureCurrentIsolate();
+}
+
 void UIMonoState::SetWindow(std::unique_ptr<Window> window) {
   window_ = std::move(window);
 }

engine/src/lib/ui/ui_mono_state.h

  public:
   static UIMonoState* Current();
 
+  static bool EnsureCurrentIsolate();
+
   Window* window() const { return window_.get(); }
 
   const TaskRunners& GetTaskRunners() const;

engine/src/lib/ui/window/window.cc

 }
 
 UIWIDGETS_API(Mono_Handle) Window_instance() {
+  if (!UIMonoState::EnsureCurrentIsolate()) {
+    return nullptr;
+  }
   return UIMonoState::Current()->window()->mono_window();
 }
 

engine/src/runtime/mono_state.cc

   return isolate_data->get();
 }
 
+bool MonoState::EnsureCurrentIsolate() {
+  return Mono_CurrentIsolate() != nullptr;
+}
+
 MonoState* MonoState::Current() {
   auto isolate_data =
       static_cast<std::shared_ptr<MonoState>*>(Mono_CurrentIsolateData());

engine/src/runtime/mono_state.h

 
   static MonoState* From(Mono_Isolate isolate);
   static MonoState* Current();
+  static bool EnsureCurrentIsolate();
 
   std::weak_ptr<MonoState> GetWeakPtr();