E010 - Nested groups not allowed

Cause

A slide group entry inside slides: contains another group entry rather than slide ids/entries. Groups are a single level deep.

Example

{ slides: [
  { group: "outer", slides: [
    { group: "inner", slides: ["a"] }   // nested group not allowed
  ]}
]}

How to fix

Flatten the groups. Each group's slides: list must contain only slide ids (strings) or slide objects, not other groups.