fix(flow-design): fix save button at flow design page's issue (#288)

PR checklist

This would fix the issue #288 , which caused by targeting the wrong element.

Looks like there's an 'overlay' suppose to be triggered, however, the target element exist neither.

Further improvement can be made by adding tests cases to prevent the issue from occurring.

Docs

[x] If you added new code; is it self documenting? (all the variable names clear, comments added where appropriate)
[x] If you touched existing code; did you refactor existing naming, added comments where necessary?

Tests

[ ] If your ticket adds new functionality; have you added unit tests to verify behaviour?
[ ] If your ticket alters user interaction with the UI; have you added integration tests?
[ ] Do your tests check for every convievable user behabviour? (all behaviours the user can perform)

Typing

Styling

[ ] If new styles were created; Did you refactor/add more styles in css-in-js style?