404 error when creating a milestone

Steps to reproduce:

1 - Create a group without a project
2 - Go to the group page and try to create a milestone

Result: You will get a 404 error

@DouweM

Added ~12980 label

Added ~155950 label

Milestone changed to 8.7

Reassigned to @felipe_artur

mentioned in merge request !3481 (merged)

Status changed to closed by merge request !3481 (merged)

mentioned in commit 074c2393