fix error when setting production editor

app/components/BookBuilder/BookBuilder.jsx

@@ -76,6 +76,8 @@ export class BookBuilder extends React.Component {
       return _.includes(productionEditorsTeam.members, u.id)
     })
 
+    if (_.isEmpty(productionEditors)) return
+
     this.setState({ productionEditor: productionEditors[0].username })
   }