avoiding assertion on macOS in wxOwnerDrawnComboBox::DoClear

wxTextEntry on macOS is calling GetEditableWindow to arrive at the native text entry view, which in this case fails, so route things directly

src/generic/odcombo.cpp

@@ -988,7 +988,14 @@ void wxOwnerDrawnComboBox::DoClear()
 
     // There is no text entry when using wxCB_READONLY style, so test for it.
     if ( GetTextCtrl() )
+    {
+#ifdef __WXOSX__
+        // this has to be rerouted to the text control explicitly on macOS
+        GetTextCtrl()->Clear();
+#else
         wxTextEntry::Clear();
+#endif
+    }
 }
 
 void wxOwnerDrawnComboBox::Clear()