Fix start reminder users slicing

Ghost User
2019-12-13T15:41:54+01:00

Closes #989

#### Previous behaviour
Steps to reproduce:

1. Create an event with participants
2. Crash on save

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

1. Create an event with participants
2. No crash on save

Closes #989 (closed)

Previous behaviour

Steps to reproduce:

1. Create an event with participants

2. Crash on save

New behaviour

Steps to validate that it works:

1. Create an event with participants

2. No crash on save