Comment # 2 on bug 1078716 from Jason Evans

(In reply to Ancor Gonzalez Sosa from comment #1)
> The problem is that GuidedSetup.initial was unable to create a valid
> proposal (likely because /dev/vda is too small for Kubic requirements). But
> instead of reporting an "storage proposal not possible" error, it crashed
> when trying to assign the non-existing proposal here
> 
> https://github.com/yast/yast-storage-ng/blob/master/src/lib/y2storage/
> clients/partitions_proposal.rb#L163

Cool, thanks for the info. I increased the qcow to 40G and it's working now.
Please close the bug.