If you delete some category or beans from the default Palette content (either accidentally or intentionally), you can restore it as follows:

  1. Exit the IDE.
  2. Go to your user directory and go down to config/Palette subdir. In this directory, the files or directories deleted from the default palette content are presented as empty files with _hidden suffix in their names.
  3. Delete these _hidden files to let the palette categories and items appear again in the IDE.

Source: NetBeans User FAQ