portals and onRender

When converting our main solution to Servoy 6.0.4 I bumped into this:
A form in 'table view (locked) has a portal with a field based on a relation. The reason for this is that any record in the table can have more then one (sub)record. Only a portal can diplay this (worked perfect in Servoy 5).
However under Servoy 6 at smartclient startup in de Console a warning appears:

Login mislukt

java.lang.ClassCastException: com.servoy.j2db.smart.dataui.PortalComponent cannot be cast to com.servoy.j2db.dataprocessing.IDisplayData

And then the form gets al sorts of renderproblems, even the parentform that holds the Table View form in a Tabpanel. Toatally not usable.
Can anyone think of a solution?
Is this a bug?

My workaround for the moment is to use a label that has a calcuated field as dataprovider. Works fine.