Uncaught (in promise) Error: GraphQL error: Cannot read property 'id' of null

Steps to reproduce:

1. Login as a new user (hasn't entered their email yet)
2. Select a trip from the home page
3. Add trip to cart
4. Go to cart
5. Click Book All

It will explode with: Uncaught (in promise) Error: GraphQL error: Cannot read property 'id' of null.

Looking at the error more closely reveals that we are caching the guest user somewhere. The error comes back from

```
// src/datasources/user.js:35
const userId = this.context.user.id;
if (!userId) return;
```

This means the user context is null