I fixed it, for the most part.
Line 25 of king-text.php is:
include_once(widgets.php’);
Before you upload the file to your plugins folder, get rid of that line of code. I imagine the redundancy of including that file (which doesn’t work even if you direct it to the correct wp_includes folder) is what’s breaking it.
I deleted that line, uploaded the php file, activated the plugin, and it works. There is however an ugly little bug when editing the widget settings, where the settings background cuts off – but on the blog itself, it seems to work fine.