Guided rule editor: rule cannot be saved when using expression editor

Description of problem:
If you use the expression editor to build a field constraint, the guided rule cannot be saved. You always get an Error modal with message like this:
'Fact type "Fact1" has a field "field1" that has an operator set, but no value. Please add a value or remove the operator.'
This happens no matter if the expression is valid or not.

(Apart from this little bug, that safeguard is a very nice feature.)

Version-Release number of selected component (if applicable):
BPMS 6.0.2 ER1

Steps to Reproduce:
1. In a guided rule, add some 'Fact1' to the 'when' section, bind it to variable 'f1'.
2. Add another 'Fact1', and add a constraint on some 'field1' to it.
3. Select operator (e.g. 'equal to'), choose the expression editor to enter the 'value'.
4. Choose 'f1', then 'field1'.
5. Validate and try to save the rule.

Actual results:
Validation is okay, saving results in Error modal with message 'Fact type "Fact1" has a field "field1" that has an operator set, but no value. Please add a value or remove the operator.'
Rule is not saved.

Expected results:
Rule is saved (and validation results are still okay).