My solution:
In case anyone runs into this problem in the future. I had renamed one of my widgets sometime during my project and then got this error during packaging. In Panteonio’s example, try to create a widget in that folder called “screen_master” and see if it lets you. It didn’t for me, saying theres already a file with that name, so I went into the project root folder → content and found a file with the old name was still hanging around. I deleted it and it packaged fine.