PanelPopup bewerbAKo = new PanelPopup(String.format("%s (%s)", name, desc), koIcon, new JScrollPane(SwingUtils.withPadding(koRoundPanel, 16)));
AbstractButton remove = bewerbAKo.addButton(AllIcons.Action.Delete.get(), "Bewerb entfernen");
...
tabFrame.addTab((TabFramePopup) bewerbAKo, name, koIcon, Alignment.SOUTH_WEST);
tabFrame.revalidate();
tabFrame.repaint();
Add a tab like this and change the LaF settings (using the Laf setting menu provided by darklaf) and the added buttons will disappear.