Fixed a crash in the properties editor

This crash happened sometimes when clicking on the
node elements (e.g. "Boxes" or "Instances") of the
object tree, the node was the second or further one
and the category clicked at was containing one
item only.

src/layui/layui/layPropertiesDialog.cc

@@ -343,9 +343,9 @@ PropertiesDialog::current_index_changed (const QModelIndex &index, const QModelI
           m_object_indexes.push_back (oi);
         }
 
-      } else {
+      } else if (mp_properties_pages [m_index]->count () > 0) {
 
-        m_object_indexes.push_back (size_t (mp_tree_model->object_index (index)));
+        m_object_indexes.push_back (0);
 
       }