Web IDE does not work on some branch name

Summary

If a branch is named as following project_group/project_name#123, the Web IDE is unable to edit files on that branch.

Steps to reproduce

• Create a branch named project_group/project_name#123
• Edit a file on that branch with Web IDE

What is the current bug behavior?

Gitlab show that banner upon running Web IDE:

What is the expected correct behavior?

Web IDE should be able to edit any file on any branch

Results of GitLab environment info

gitlab 12.4.0-ee on-premise