Source is not available error while creating MR + branch

Summary

Misleading error message while creating merge request from a non-master branch

Steps to reproduce

• Create a branch, say 1.0-release, from master
• Create a new issue
• On the issue page, click on Create merge request, and select "Create merge request and branch"
• In the source box, start typing 1.0-release, it will get autofilled
• There is an error message at the bottom of the box saying "Source is not available"
• This error message stays there until I click inside the "Branch name" box and back in the "Source (branch or tag)" box, and remove and add the last character of the source branch/tag.
• Finally the message goes away and it says "Source is available"

Example Project

NA

What is the current bug behavior?

Error message. This is harmless and goes away after I click inside the other box, but it's still annoying and misleading.

What is the expected correct behavior?

No error message should show up

Relevant logs and/or screenshots

Output of checks

(If you are reporting a bug on GitLab.com, write: This bug happens on GitLab.com)

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:	2.3.6p384
Gem Version:	2.6.13
Bundler Version:1.13.7
Rake Version:	12.3.0
Redis Version:	3.2.11
Git Version:	2.14.3
Sidekiq Version:5.0.5
Go Version:	unknown
GitLab information
Version:	10.4.4
Revision:	1d22950
Directory:	/opt/gitlab/embedded/service/gitlab-rails
DB Adapter:	postgresql
URL:		----removed----
HTTP Clone URL:	----removed----
SSH Clone URL:	----removed----
Using LDAP:	no
Using Omniauth:	yes
Omniauth Providers: saml
GitLab Shell
Version:	5.11.0
Repository storage paths:

• default: /data/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 >= 5.11.0 ? ... OK (5.11.0)
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: ...
10/5 ... ok
5/6 ... ok
10/7 ... ok
10/8 ... 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
Checking Reply by email ...
IMAP server credentials are correct? ... no
Exception: undefined method `message' for #String:0x00007f2bd34cdca8
Init.d configured correctly? ... skipped
MailRoom running? ... skipped
Checking Reply by email ... Finished
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: ...
10/5 ... yes
5/6 ... yes
10/7 ... yes
10/8 ... yes
Redis version >= 2.8.0? ... yes
Ruby version >= 2.3.5 ? ... yes (2.3.6)
Git version >= 2.7.3 ? ... yes (2.14.3)
Git user has default SSH configuration? ... yes
Active users: ... 6
Checking GitLab ... Finished

Possible fixes

(If you can, link to the line of code that might be responsible for the problem)