Customer Managed Group Member returns null customer if registered on different channel

[mschipperheyn]

Which plugin/repository is the issue about?
customer-managed-group

Describe the bug
If you (accidently) create a customer on the default channel and associate this customer to a customer on another channel through customer managed group, the customer relation will be null when executing getCustomerManagedGroup

To Reproduce
Steps to reproduce the behavior:

1. Add a customer on a non default channel
2. Add another customer on the default channel
3. Put the customers in a customer managed group
4. Load either one of the groups on the respective channel

Expected behavior
The customer member should not be retrieved at all as part of the customer managed group

Screenshots

Environment

• NodeJS version
• Vendure version: [e.g. 1.7.1]: 2.3.0
• Plugin version [e.g. 3.1.1]
• Database [e.g. MySQL]
• Any other version numbers from the environment you're using

Additional context