Missing selection set for object GraphQL+Apollo error

Solution is to add fields to the query (vs. declaring the top-level object you want to fetch without specifying the fields to fetch). If you have something like:

{
  popups @client {
    id
    dialog
  }
}

you must declare some fields to fetch inside dialog, for example id:

{
  popups @client {
    id
    dialog {
      id
    }
  }
}

Answer to the actual problem seems lay in the query. Initially Apollo client was not validating types for @client queries/mutations so your mutation could look like you wrote it in the question:

mutation AlertOpenDialog($type: String!) {
  openDialog(type: $type) @client
}

the correct way of writing above is to specify (select) all the simple type (scalar) fields you wanna get in a response. So in regard to the @Vic answer the mutation should look more like:

mutation AlertOpenDialog($type: String!) {
  openDialog(type: $type) @client {
    dialog {
      id
    }
  }
}