Cannot return null for non-nullable field Query.login (GraphQL, React)

Question

I've been working on this issue all day on setting up authorisation for my app and realised the issue of this error

Cannot return null for non-nullable field Query.login

Stems from this bit of code here in AuthPage.tsx.

return res.json();

I'm not quite sure the best way to process the data after it's come back from GraphQL (which it's coming back with a 200 response which is great and the information).

This is the full authorisation component called AuthPage.tsx

import React, { Component } from 'react';
import './AuthPage.scss';

export class AuthPage extends React.Component {
    state = {
        isLogin: true
    }

    constructor(props: any) {
        super(props);
        this.emailEl = React.createRef();
        this.passwordEl = React.createRef();
    }

    emailEl: React.RefObject<any>;
    passwordEl: React.RefObject<any>;

    switchModeHandler = () => {
        this.setState((prevState: any) => {
            return {isLogin: !prevState.isLogin}
        })
    }


    submitHandler = (event: any) => {
        event.preventDefault();
        const email = this.emailEl.current.value;
        const password = this.passwordEl.current.value;

        if (email.trim().length === 0 || password.trim().length === 0) {
            return
        }

        let requestBody = {
            query: `
                query {
                    login(email: "${email}", password: "${password}") {
                            userId
                            token
                            tokenExpiration
                    }
                }
            `
        };

        if (!this.state.isLogin) {
            requestBody = {
                query: `
                mutation {
                    createUser(id: "", email: "${email}", password: "${password}") {
                        id
                        email
                        password
                    }
                }
                `
            }
        }

        fetch('http://localhost:4000/graphql', {
            method: 'POST',
            body: JSON.stringify(requestBody),
            headers: {
              'Content-Type': 'application/json'
            }
          })
            .then(res => {
              if (res.status !== 200 && res.status !== 201) {
                throw new Error('Failed!');
              }
              return res.json();
            })
            .then(resData => {
              console.log(resData);
            })
            .catch(err => {
              console.log(err);
            });
        };

    render() {
        return(
            <form className="login" onSubmit={this.submitHandler}>
                <div className="form-control">
                    <label className={"label"} htmlFor={"email"}>Email:</label>
                    <input type="email" placeholder={"Email address"} id={"email"} ref={this.emailEl}/>
                </div>
                <div className="form-control">
                    <label className={"label"} htmlFor={"password"}>Password:</label>
                    <input type="password" placeholder={"Enter password"}  ref={this.passwordEl}/>
                </div>
                <button type={"submit"} className={"button"}>Login</button>
                <button type={"button"} className={"button"} onClick={this.switchModeHandler}>Switch to {this.state.isLogin ? 'Signup' : 'Login'}</button>
            </form>
        )
    }
}

export default AuthPage;

The Schema

module.exports = `
  type User {
    id: ID!
    email: String!
    password: String
  }

  type AuthData {
    userId: ID!
    token: String!
    tokenExpiration: Int!
  }

  input UserInput {
    email: String!
    password: String!
  }

  type Query {
    user(id: String!): User
    users: [User]
    login(email: String!, password: String!): AuthData!
  }

  type RootMutation {
    createUser(userInput: UserInput): User
  }

  type Mutation {
    createUser(id: String, email: String!, password: String!): User
    editUser(id: String, email: String): User
    deleteUser(id: String, email: String): User
  }
`;

My goal is to return the GQL response from the server and format it correctly into a response then return AuthData

but most importantly getting rid of this error Cannot return null for non-nullable field Query.login.