The widget that causes this issue, is brand new, freshly created by Yeoman, and I added 1 folder in the lib folder. I can't find the issue here and I'm stuck.
After adding gesture from dojox, I get the error BUT when I check the build-report.txt I don't see any errors:
Process finished normally
errors: 0
warnings: 64
build time: 3.984 seconds
We can't deploy a production system because of this. The project runs fine if you uncheck "Bundle widgets when running locally"