Widget names need to have a hyphen in them, my guess is this is the issue. This isn't something that we enforce, this is actually per the HTML5 spec for custom elements.
If you do have a hyphen and are still running in to this issue, then you may be trying to create a widget starting with 'ts', which we've reserved for our system widgets.
We'll clean up that error message.
Let me know if this doesn't solve your issue and we'll dig deeper.