Update button parent reference in busy_dialog

Author: gammasoft71

examples/xtd.forms.examples/dialogs/busy_dialog/busy_dialog.cpp

@@ -7,7 +7,7 @@ class form1 : public form {
 
     button1.auto_size(true);
     button1.location({10, 10});
-    button1.parent(*this);
+    button1.parent(self_);
     button1.text("Do something...");
     button1.click += delegate_ {
       auto dialog = busy_dialog {};