Initialize wxPGChoices object properly in its copy ctor if source object has no data.

Internal data must be always initialized whether the source object contains choices data or not.

Closes #16855

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@78484 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775

include/wx/propgrid/property.h

@@ -858,6 +858,10 @@ public:
             m_data = a.m_data;
             m_data->IncRef();
         }
+        else
+        {
+            Init();
+        }
     }
 
     /**