Remote repository mirroring fails

Summary

When trying to use the "Push to a remote repository" feature, the action fails with the error message undefined method 'id' for nil:NilClass.

Steps to reproduce

1. Create a GitHub repository and make sure the wiki is enabled
2. Create at least one wiki page
3. Configure the "Push to a remote repository" feature to mirror the wiki repo to the main project repo
4. Instead of waiting for the next sync, enforce it by clicking the Update Now button
5. See the error mentioned in the summary of this issue

What is the current bug behavior?

See summary of the issue.

What is the expected correct behavior?

The repository should be mirrored as expected by the "Push to a remote repository" feature.

Output of checks

This bug happens on GitLab.com (tested on 9.0 release)

Possible fixes

As I'm not sure why the feature fails, no fix can be proposed at the time of handing in this issue.