Hard check for JTable in ColumnModelFactory.onNodeCompleted prevented columnModel() to be used in a noparent()-block

If you want to use columnModel() outside of a table-node, e.g. to reuse it in multiple tables or to add to the table via code, it should be able to use it in a noparent-block.
This had been prevented because 'parent.columnModel = node' throws a MPE if the parent has no columnModel property.
This had been fixed by only setting the property when the parent is a JTable.