Comment # 2 on bug 1209162 from Stefan Hundhammer

https://github.com/yast/yast-s390/blob/master/src/lib/y2s390/dasds_reader.rb#L172-L176

> def use_diag?(dasd)
>   diag_file = "/sys/#{dasd.sysfs_id}/device/use_diag"
>   use_diag = Yast::SCR.Read(Yast.path(".target.string"), diag_file) if File.exist?(diag_file)
>   use_diag.to_i == 1
> end

If the file does not exist, "use_diag" will remain nil, and "use_diag.to_i"
will throw a nil.NilClass exception because "use_diag.to_i" will evaluate to
"nil.to_i".