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It is possible to choose icon (disabledIcon, etc.) directly from the comboBox in the Properties window. I think it should be possible to "unset" it the same way, I mean like adding an option <none> into the comboBox. Moreover, it would correspond to other Properties, that could be set to some value or set to "none" (like nextFocusableComponent for example).
I agree, this makes sense.
Fixed together with http://hg.netbeans.org/jet-main/rev/9582c982eeac.