When wxPGProperty is un-attached from wxPropertyGrid, keep its 'default' cell references invalid/NULL (fixes #12552)

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@65788 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775

src/propgrid/propgridpagestate.cpp

@@ -1934,6 +1934,8 @@ void wxPropertyGridPageState::DoDelete( wxPGProperty* item, bool doDelete )
     // We can actually delete it now
     if ( doDelete )
         delete item;
+    else
+        item->OnDetached(this, pg);
 
     m_itemsAdded = 1; // Not a logical assignment (but required nonetheless).