This appears to be a regression from this commit: https://github.com/phpbb/phpbb3/commit/0e02f5cb0b7f615f1eadd6606a89bfd1b28f0d0d#L1L1655
The new code for removing components doesn't check to make sure that there is another active style to replace it. Thus, users can remove the last one and be presented with a General Error about being unable to retrieval the nonexistent style data, breaking their board.