[OSX] until window is shown FrameSize is null.

native/Avalonia.Native/src/OSX/WindowBaseImpl.mm

@@ -191,9 +191,8 @@ HRESULT WindowBaseImpl::GetClientSize(AvnSize *ret) {
         if (ret == nullptr)
             return E_POINTER;
 
-        auto frame = [View frame];
-        ret->Width = frame.size.width;
-        ret->Height = frame.size.height;
+        ret->Width = lastSize.width;
+        ret->Height = lastSize.height;
 
         return S_OK;
     }
@@ -206,9 +205,15 @@ HRESULT WindowBaseImpl::GetFrameSize(AvnSize *ret) {
         if (ret == nullptr)
             return E_POINTER;
 
-        auto frame = [Window frame];
-        ret->Width = frame.size.width;
-        ret->Height = frame.size.height;
+        if(Window == nullptr){
+            ret->Width = 0;
+            ret->Height = 0;
+        }
+        else {
+            auto frame = [Window frame];
+            ret->Width = frame.size.width;
+            ret->Height = frame.size.height;
+        }
 
         return S_OK;
     }

src/Avalonia.Native/WindowImplBase.cs

@@ -117,7 +117,7 @@ namespace Avalonia.Native
                 if (_native != null)
                 {
                     var s = _native.FrameSize;
-                    return new Size(s.Width, s.Height);
+                    return s.Width == 0 && s.Height == 0 ? null : new Size(s.Width, s.Height);
                 }
 
                 return default;