Add null check,

Customer was seeing a crash in this method, don't have a repro but other uses of `_parent` are surrounded by a null check so this makes sense I think.

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

@@ -281,6 +281,9 @@
 
 - (void)windowDidBecomeKey:(NSNotification *_Nonnull)notification
 {
+    if (_parent == nullptr)
+        return;
+        
     _parent->BringToFront();
     
     dispatch_async(dispatch_get_main_queue(), ^{