500 error on activity feed, when deleted branch or repo is pushed was done

Summary: HTTP 500 error on activity feeds (dashboard, project view) in v7.3
Steps to reproduce: Remove existing branch or push to repo and try to access the dashboard or activity feed.
Expected behavior: Show the activity feed.
Observed behavior: An 500 error as the branch seems to need to be fetched and fails.
Relevant logs:

ActionView::Template::Error (undefined method `[]' for nil:NilClass):

1: - if show_last_push_widget?(event)

2: .event-last-push

3: .event-last-push-text

4: %span You pushed to

app/models/event.rb:186:in `branch?'

app/models/event.rb:257:in `last_push_to_non_root?'

app/helpers/application_helper.rb:125:in `show_last_push_widget?'

app/views/events/_event_last_push.html.haml:1:in `_app_views_events__event_last_push_html_haml___2437686736074306650_38485100'

app/views/groups/show.html.haml:4:in `_app_views_groups_show_html_haml__4009651433334491628_21676780'

app/controllers/groups_controller.rb:42:in `show'

Version of GitLab you are running: GitLab 7.3.0 76eaf55d
Describe your setup:

System information
System:		Ubuntu 14.04
Current User:	git
Using RVM:	yes
RVM Version:	1.25.27
Ruby Version:	2.1.2p95
Gem Version:	2.2.2
Bundler Version:1.6.2
Rake Version:	10.3.2
Sidekiq Version:2.17.0

GitLab information
Version:	7.3.0
Revision:	76eaf55
Directory:	/home/git/gitlab
DB Adapter:	mysql2
URL:		https://dev.*.eu
HTTP Clone URL:	https://dev.*.eu/some-project.git
SSH Clone URL:	git@dev.*.eu:some-project.git
Using LDAP:	no
Using Omniauth:	no

GitLab Shell
Version:	2.0.0
Repositories:	/home/git/repositories/
Hooks:		/home/git/gitlab-shell/hooks/
Git:		/usr/bin/git