Fix ~wxDataViewCtrl assert when running under JAWS

Accessible object must be destroyed as soon as possible, because
otherwise a screen reader may try to query it before wxWindow destructor
removed it, but after ~wxDataViewCtrl destructor finished. The
wxACC_EVENT_OBJECT_DESTROY notification causes exactly that under JAWS.

src/generic/datavgen.cpp

@@ -4731,6 +4731,7 @@ wxDataViewCtrl::~wxDataViewCtrl()
     m_colsBestWidths.clear();
 
 #if wxUSE_ACCESSIBILITY
+    SetAccessible(NULL);
     wxAccessible::NotifyEvent(wxACC_EVENT_OBJECT_DESTROY, this, wxOBJID_CLIENT, wxACC_SELF);
 #endif // wxUSE_ACCESSIBILITY
 }