This is probably due to an error in your ShiftCalendar.xml file.
I deleted the contents of my manually created xml file with the xml from the Crash Course (at the end of the “Extending your UI” lecture).
Then during refresh I got an error message about an invalid “name” attribute.
To solve that error, I opened the xml file, and changed line 40 from <attributeType name="string"/> to
<attributeType name="String"/>
Then I rebuilt the widget, copied it into the widgets directory, and after a refresh it showed up