I have created that for an older version of the webapp builder but didnt have time to update date it.
Here is the live version ArcGIS Web Application , you can right click graphics and do some modifications.
Here is the forum post for the widget:https://community.esri.com/docs/DOC-3256
You may also want to look at Jeremie's eDraw widget - which not only edits but can save the drawing to a file, to re-load into the site and edit later, even on a different machine.
We love both of these widgets.