"Create merge request and branch" doesn't work after failed option

Summary

When attempting to create an MR and branch from within an issue on GitLab.com, if you enter an incorrect "Source" branch name at any point, you lose the "Create merge request" button.

Steps to reproduce

1. From within an issue, select the dropdown next to "create merge request"
2. Enter Branch name
3. Enter an incorrect source branch or tag name
4. Fix the source branch to something eligible
5. Notice the "Create merge request" button never becomes available to select even though the Source is available.

What is the current bug behavior?

The "Create merge request" button never becomes available to select even though the Source is available.

What is the expected correct behavior?

After an available source is entered, the "Create merge request" button should be available to select.

Relevant logs and/or screenshots

video of issue here: https://www.loom.com/share/2440050999b64355ab918a5c014344e3

Affected user support tickets:

• https://gitlab.zendesk.com/agent/tickets/126239

/cc @jramsay