Missing PV preview viewer if it is closed when docked

[kswang1029]

Describe the bug
When we request PV preview, the PV preview viewer will show up as a floating widget. If we dock it and close it, then click "Start preview" button, the PV preview viewer never comes back. If the PV preview viewer stays floating and being closed, things are just fine.

To Reproduce
Steps to reproduce the behavior:

1. load a cube and request pv preview.
2. dock the PV preview widget
3. close the PV preview widget
4. click "Start preview" button. We expect to see the PV preview widget shows up but it does not.

Expected behavior
In this use case, the PV preview widget should come back.

Platform info (please complete the following information):

• OS [e.g. macOS Monterey]: macOS Ventura
• Browser [e.g. chrome, safari, electron app]: electron
• Browser version [e.g. 22]:
• Backend branch [e.g. dev, v3b2 release]: dev. v4-stable
• Frontend branch [e.g. dev, v3b2 release]: dev. v4-stable

[kswang1029]

fixed via PR#2257