Fix crash when ForeignKey is left empty on model save

Sébastiaan Versteeg requested to merge fix/organiser-save-crash into master

Closes #866 (closed)

Previous behaviour

Steps to reproduce:

  1. Create a new event without value in the organiser field
  2. Get a RelatedObjectDoesNotExist error

New behaviour

Steps to validate that it works:

  1. Create a new event without value in the organiser field
  2. Get the form with the right error messages

Merge request reports