Able to create merge request from API with nonexistent target branch

Summary

You are able to create a merge request from the API with a target_branch that doesn't exist. When navigating to the MR in the UI, you see the error below.

Steps to reproduce

  1. Create MR through the API with target_branch that doesn't exist.
  2. Navigate to MR in UI

What is the current bug behavior?

No errors or messages are returned from the API. In the UI, the below error is seen.

What is the expected correct behavior?

An error message should be returned from the API that the target_branch does not exist.

Relevant logs and/or screenshots

mr-no-branch

GitLab 11.0.2-ee

Customer ticket -> https://gitlab.zendesk.com/agent/tickets/99660 (internal use)