Steps to reproduce

1. Open Blue Ocean
2. Open a Pipeline
3. Edit the Pipeline in the Blue Ocean Pipeline Editor
4. Make a change to the Agent drop-down
5. Click "Go to Classic" button in top right of page

Expected result:
Dialog "There are unsaved changes, discard them?"
Navigate to classic configuration page for the current job or to the job main page

Actual result:

HTTP ERROR 404
Problem accessing /job/pipeline-editor. Reason:
Not Found