Guided Rule Editor: Expression editor not available for a fact

Description of problem:
Expression editor cannot even be selected - see 'Steps to Reproduce' and 'Actual results'.

Version-Release number of selected component (if applicable):
6.1 ER3

Steps to Reproduce:
1. Create a guided rule, add a simple condition: There is a Person.
2. Modify constraints for Person - select Expression editor.

Actual results:
Pop-up ('Modify constraints for Person') does not disappear. The previously added condition (Person( )) disappears from the guided editor. As well as everything below the WHEN section.
If you close the pop-up, you cannot re-add the condition - you get the pop-up prompting to choose a condition, but nothing is added.
Source tab is empty, rule cannot be closed.

Expected results:
Expression editor can be chosen and works, rule can be reopened, nothing disappears that should not.

Additional info:
This also seems to affect rules using subfields - e.g. the examples from bug 986000. Such rule can be saved, validates fine, but when closed, cannot be reopened. The editor then stays empty, and the rule cannot be closed again. The source of the rule is still valid though.
(Constraint on a subfield not added via expression editor, when the rule is reopened, the constraint is done in the expression editor.)