Custom sidebar once unpinned never come back

[conanliuhuan]

Describe the bug
Can NOT reopen an unpinned sidebar.
Unpin a custom sidebar from toolbar, close it. It can Not reopen.

To reproduce

1. Follow "Gutenberg handbook / tutorials" to registered a custom sidebar.
2. Open editor (Write a new post)
3. Open your sidebar from toolbar
4. Click the "star" icon (unfilled) to unpin your sidebar from toolbar
5. Click the close icon near star icon
6. Refresh your browser
7. Your custom sidebar NEVER come back

Expected behavior
There should be a way to reopen the sidebar. But I can't find it anywhere.
First I expect it should be in options icon (top right on the screen), But nothing there.

Screenshots
If applicable, add screenshots to help explain your problem.

Desktop (please complete the following information):

• OS: Windows
• Browser Firefox
• Version 68.0.1

Additional context

• Please add the version of Gutenberg you are using in the description.
• To report a security issue, please visit the WordPress HackerOne program: https://hackerone.com/wordpress.

[conanliuhuan]

Problem solved:
https://github.com/WordPress/gutenberg/blob/master/packages/e2e-tests/plugins/plugins-api/sidebar.js#L85-L102