Specious error message when successor does not exist

[Rob Donnelly]

Expected Error

Something like

Job 'absent_parent' does not exist for 'child' job @ error.jr:1

Actual Error

.../jobrnr/lib/jobrnr/job/definition.rb:19:in `block in initialize': undefined method `successors' for nil:NilClass (NoMethodError)

Reproduce

error.jr

job :child, :absent_parent do
  execute 'true'
end

jobrnr error.jr

[Rob Donnelly]

Related to #5