Hi,

I created a new widget. It was written below the name that "Use only lowercase alphanumeric characters, underscores (_), and hyphens (-)" so I named it to my_widget.
After the settings I went to the Structure->Blocks menu, but I did not see my widget.
I tried to create it again with the same settings, and uninstalled the module too, but nothing helped.

Then I renamed it my-widget (so I just changed the underline character) and it appeard in Blocks menu immediately.
I have not looked at the code yet and I am not a coder really, but I think it is bug. Maybe it does not find the _ character in the Blocks.

I hope it can help a little bit. Just tell me if I was wrong or I did something uncorrectly.