GraphQLError Undefined location on exception

[keletsomolefe]

Describe the bug
On a query with an exception, locations is set to null on 3.0.0-beta.3 this is because raw['locations'] gives Map of type Map<String, dynamic> while the code is checking for Map<String, int>

https://github.com/zino-app/graphql-flutter/blob/d17b57b52d3cb985a8c8d7025a242bbcddecabfb/packages/graphql/lib/src/exceptions/graphql_error.dart#L33

To Reproduce
Steps to reproduce the behavior:

1. Create a query that will return you errors which have locations
2. Check the query result's exception
3. e.g. result.exception.graphqlErrors[0].locations, result is an instance of 'QueryResult'

[micimize]

This is outdated / likely solved by using gql_links in 4.0.0-beta.1 (#648)