If you make a mistake such as not adding a step to a stage, you will correctly see a validation warning, and a message like the attached.
Presumably this is coming back via PMD - so in all likelihood this ticket should be transferred to that.
'Array has 0 entries, requires minimum of 1'
When ideally in the users terms it would say "A stage requires at least one step". Ideally not the UI job to translate, but just putting it here for discussion.