Thanks for getting back to me Garo. As a developer I know it's frustrating when you can't re-create the issue a user reports. Thanks for helping me.
I tried all recommended steps (with one caveat) and I'm still having this issue.
The caveat is that when I repair visual studio 2017 it automatically removes the appbuilder extension.
Immediately after the repair is done the error goes away, but the appbuilder extension is also gone. When I re-install the appbuilder extension the error returns.
Any other ideas? Are there any error logs that would give us better insight into what's actually going wrong?