[OSX] Dont crash if Window is null when Close is called.

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

@@ -195,7 +195,11 @@ public:
     {
         @autoreleasepool
         {
-            [Window close];
+            if (Window != nullptr)
+            {
+                [Window close];
+            }
+            
             return S_OK;
         }
     }