upgrading project from 6 to 7 and later makes variables as undefined

1. set up project in 6.0
2. create workflow with workflow variables
3. create at least a node
4. access the workflow variables from the node
5. upgrade to 7
6. validation errors appear (see validationErrors.png)
7. checking the form all seems ok (see worflowVariablesInNodeForm.png)

The only workaround found is to manually select the variables and drag them into the widget (see dragAndDrop.png)

This can be painful when upgrading with many variables.