Fixed creating group membership without filling in required field 'member' or 'group'

Ghost User
2019-11-13T21:59:18+01:00

Closes #910

#### Previous behaviour
Steps to reproduce:

1. Create a group membership without filling in either a name or a group
2. breakage

#### New behaviour
Steps to validate that it works:

1. Create a group membership...

Closes #910 (closed)

Previous behaviour

Steps to reproduce:

1. Create a group membership without filling in either a name or a group

2. breakage

New behaviour

Steps to validate that it works:

1. Create a group membership without filling in either a name or a group

2. Receive error message