I tried making Slice Menu of 'navigation menu' to know how this module works.
Then I clicked 'Reset to defaults' in '/admin/settings/slicedmenu' but the situation doesn't go back to defaults.
In '/admin/build/block' page, there remain some fragments like _level_0, _level_1, _level_2, _level_3,_level_4, _level_5,
and I cannot delete these blocks.