As I see the
DialogComponentFileChooser is not final, so it is possible to override the
validateSettingsBeforeSave() method. It is still possible with this solution to accept empty String there (with a red warning sign; swallowing the exception). This works fine till there is no previously selected file in that field. After that in the next configuration attempt when I try to delete the whole content (selecting all, then [del]) it seems to be no change in the model. What else should I override to be able to accept no file selection?
Thanks in advance, gabor
PS.: Actually it would be useful if the valid extensions could be groupable (like: [.txt|.TXT|.xml|.html], [.csv|.CSV],  would be three groups with cardinalities: 4, 2, 1).