Why am I getting "Cannot return null for non-nullable field User.sentEmail."?

Question

I understand that I would receive this error if the field I am trying to call is null, but when I do a create email, that email's id does gets saved in the user. But when I try to query that user using the following query

query all {
  users{
    sentEmail{
      sender
      emailbody
      recipient
    }
  }
}
}

I get the error "Cannot return null for non-nullable field User.sentEmail." If there is a id in sentEmail, why am I getting this error

Resolver

const { Email, User } = require('../models')

const resolvers = {
    Query : {
        users : async () => {
            return await User.find({}).populate('emails');
        },
        emails : async (parent, { email }) => {
            const params = email ? { email } : {};
            return await Email.find(params).sort({ sentDate: -1 });
        },
    },
    Mutation : {
        addemail : async (parent, {sender, recipient, subject, emailbody}) => {
            const newemail = await Email.create({ sender, recipient, subject, emailbody});

            await User.findOneAndUpdate(
                { email: newemail.sender },
                { $addToSet : { sentEmails: newemail._id }}
            )

            await User.findOneAndUpdate(
                { email: newemail.recipient },
                { $addToSet : { receivedEmails: newemail._id }}
            )

            return newemail
        },
        addUser: async (parent, { firstName , lastName , email, password }) => {
            const user = await User.create({ firstName , lastName , email, password }, {new: true})
        },
    }
};

module.exports = resolvers

typeDefs

const { gql } = require('apollo-server-express');

const typeDefs = gql`
    type User {
        _id : ID
        firstName : String
        lastName : String
        email : String
        password : String
        sentEmail: [Email]!
    }
    
    type Email {
        _id : ID
        sender : String
        recipient : String
        emailbody : String
        sentDate : String
    }

    type Query {
        users : [User]
        emails(email: String) : [Email]
    }

    type Mutation {
        addemail(sender: String!, recipient: String!, subject: String , emailbody: String!): Email
        addUser(firstName: String!, lastName: String!, email: String!, password: String! ): User
    }
`;

module.exports = typeDefs;

Answer 1

Please check your response from the resolver. Seems it is returning the value of sentEmail field as null or sentEmail field is not present in your response json.