Problem upgrading plugins from v2 to v3: Definition object should contain 'type' property: JSON'

[stanislawosinski]

Hi,

I'm trying to update my site from Gatsby 2.24.63 to 3.13.0 and after updating the dependencies gatsby develop produces the following error:

$ gatsby develop
success open and validate gatsby-configs, load plugins - 1.793s
success onPreInit - 0.043s
success initialize cache - 0.038s
success copy gatsby files - 0.167s
success Compiling Gatsby Functions - 0.371s
success onPreBootstrap - 0.410s
success createSchemaCustomization - 0.016s
success Checking for changed pages - 0.001s
success source and transform nodes - 0.174s

 ERROR

Missing onError handler for invocation 'building-schema', error was 'Error: TypeError[.searchableFields]: Definition
object should contain 'type' property: JSON'. Stacktrace was 'Error: TypeError[.searchableFields]: Definition object
should contain 'type' property: JSON
    at TypeMapper.convertOutputFieldConfig
(C:\work\repositories\github.com\apidocs\gatsby-theme-apidocs\node_modules\graphql-compose\lib\TypeMapper.js:307:17)
   ...

The offending plugin code is probably this:

// Set the GraphQL type for LocalSearchIndex.
const createSchemaCustomization = (
  { actions: { createTypes }, schema },
  { name = DEFAULT_NAME },
) => {
  createTypes([
    schema.buildObjectType({
      name: generateTypeName(`${TYPE_INDEX} ${name}`),
      fields: {
        id: 'ID',
        engine: 'String',
        searchableFields: GraphQLJSON,
        index: GraphQLJSON
      },
    }),
  ])
};

Is there anything obvious I'm missing here? I can prepare a repo with a complete reproduction example if needed.

Thanks!

Stanislaw