Problem with retaining selected parameters from drop down lists in Mac

I'm using RapidMiner Studio 6.0 Starter edition in Mac. When I choose a parameter for an operator from a drop down list, the value is lost as soon as I change the focus. For instance, for the Set Role operator, if I choose an attribute, say XYZ, and change its role to label, the value is lost as soon as I click somewhere else (i.e., change my focus). I've found a work around that is as follows: If after selecting XYZ attrbute, I add another random character, say a space after XYZ, and then remove the added character, then the value stays. Is this a known bug or am I doing something wrong?

The problem happens with many other parameters where something is to be selected from drop down lists (e.g., Select attributes). However, it doesn't happen for entry fields, e.g., Sample (where the desired sample size is manually entered).