The relational:Native Query property doesn't take values. This doesn't seem to be possible anymore. The value that is entered is simply removed as soon as you leave the field. To reprocude:

• create a new (empty) source model
• standing in the source model right click and choose "New Child" -> "Table...".
• Enter a value for the name in the "Name" field and optionally add a column in the "Columns" tab.
• In the "Native Query" tab, enter any value, no matter wat (e.g. "test")
• Click "Ok"
• Notice that the property relational: Native Query is not filled in

You can also test this by not filling something in the Native Query tab upon creation but do it afterwards.