500 error when opening open/merged merge requests imported from Github

Summary

(Probably a follow-up of https://gitlab.com/gitlab-org/gitlab-ce/issues/38289)

I have imported a project from Github with merge requests. When I try to open the Merge requests that are in the open/merged state (the ones that are closed are opening fine), it throws a 500 error.

Steps to reproduce

Import a project from Github with some open/closed/merged pull requests.

What is the current bug behavior?

Error 500

What is the expected correct behavior?

Should display the MR.

Relevant logs and/or screenshots

==> /var/log/gitlab/gitlab-rails/production.log <==
Started GET "/eng/ninevirt/ninevirt/merge_requests/299" for 217.150.242.121 at 2017-10-10 08:33:00 +0200
Processing by Projects::MergeRequestsController#show as HTML
  Parameters: {"namespace_id"=>"eng/ninevirt", "project_id"=>"ninevirt", "id"=>"299"}
Completed 500 Internal Server Error in 164ms (Views: 126.1ms | ActiveRecord: 8.1ms)

NoMethodError (undefined method `commits_count' for nil:NilClass):
  app/models/merge_request.rb:226:in `commits_count'
  app/controllers/projects/merge_requests_controller.rb:65:in `block (2 levels) in show'
  app/controllers/projects/merge_requests_controller.rb:59:in `show'
  lib/gitlab/i18n.rb:46:in `with_locale'
  lib/gitlab/i18n.rb:52:in `with_user_locale'
  app/controllers/application_controller.rb:326:in `set_locale'
  lib/gitlab/middleware/multipart.rb:93:in `call'
  lib/gitlab/request_profiler/middleware.rb:14:in `call'
  lib/gitlab/middleware/go.rb:17:in `call'
  lib/gitlab/etag_caching/middleware.rb:11:in `call'
  lib/gitlab/request_context.rb:18:in `call'
  lib/gitlab/metrics/requests_rack_middleware.rb:27:in `call'

Output of checks

Results of GitLab environment info

Expand for output related to GitLab environment info 
System information
System:		Ubuntu 16.04
Current User:	git
Using RVM:	no
Ruby Version:	2.3.5p376
Gem Version:	2.6.13
Bundler Version:1.13.7
Rake Version:	12.0.0
Redis Version:	3.2.5
Git Version:	2.13.5
Sidekiq Version:5.0.4
Go Version:	unknown

GitLab information
Version:	10.0.3
Revision:	8895150
Directory:	/opt/gitlab/embedded/service/gitlab-rails
DB Adapter:	postgresql
URL:		https://gitlab.nine.ch
HTTP Clone URL:	https://gitlab.nine.ch/some-group/some-project.git
SSH Clone URL:	git@gitlab.nine.ch:some-group/some-project.git
Using LDAP:	no
Using Omniauth:	yes
Omniauth Providers: cas3


GitLab Shell
Version:	5.9.0
Repository storage paths:
  • default: /var/opt/gitlab/git-data/repositories Hooks: /opt/gitlab/embedded/service/gitlab-shell/hooks Git: /opt/gitlab/embedded/bin/git

Results of GitLab application Check

Expand for output related to the GitLab application check 
Checking GitLab Shell ...

GitLab Shell version >= 5.9.0 ? ... OK (5.9.0)
Repo base directory exists?
default... yes
Repo storage directories are symlinks?
default... no
Repo paths owned by git:root, or git:git?
default... yes
Repo paths access is drwxrws---?
default... yes
hooks directories in repos are links: ...
22/2 ... ok
22/3 ... ok
22/4 ... ok
21/6 ... ok
21/7 ... ok
9/8 ... ok
23/10 ... ok
23/11 ... ok
23/12 ... ok
23/13 ... ok
23/14 ... ok
16/15 ... ok
21/16 ... repository is empty
21/17 ... repository is empty
19/18 ... ok
Running /opt/gitlab/embedded/service/gitlab-shell/bin/check
Check GitLab API access: OK
Redis available via internal API: OK


Access to /var/opt/gitlab/.ssh/authorized_keys: OK
gitlab-shell self-check successful


Checking GitLab Shell ... Finished


Checking Sidekiq ...


Running? ... yes
Number of Sidekiq processes ... 1


Checking Sidekiq ... Finished


Reply by email is disabled in config/gitlab.yml
Checking LDAP ...


LDAP is disabled in config/gitlab.yml


Checking LDAP ... Finished


Checking GitLab ...


Git configured correctly? ... yes
Database config exists? ... yes
All migrations up? ... yes
Database contains orphaned GroupMembers? ... no
GitLab config exists? ... yes
GitLab config up to date? ... yes
Log directory writable? ... yes
Tmp directory writable? ... yes
Uploads directory exists? ... yes
Uploads directory has correct permissions? ... yes
Uploads directory tmp has correct permissions? ... yes
Init script exists? ... skipped (omnibus-gitlab has no init script)
Init script up-to-date? ... skipped (omnibus-gitlab has no init script)
Projects have namespace: ...
22/2 ... yes
22/3 ... yes
22/4 ... yes
21/6 ... yes
21/7 ... yes
9/8 ... yes
23/10 ... yes
23/11 ... yes
23/12 ... yes
23/13 ... yes
23/14 ... yes
16/15 ... yes
21/16 ... yes
21/17 ... yes
19/18 ... yes
Redis version >= 2.8.0? ... yes
Ruby version >= 2.3.3 ? ... yes (2.3.5)
Git version >= 2.7.3 ? ... yes (2.13.5)
Git user has default SSH configuration? ... no
Try fixing it:
mkdir ~/gitlab-check-backup-1507617390
sudo mv /var/opt/gitlab/.ssh/authorized_keys.lock ~/gitlab-check-backup-1507617390
For more information see:
doc/ssh/README.md in section "SSH on the GitLab server"
Please fix the error above and rerun the checks.
Active users: ... 11


Checking GitLab ... Finished