Group plugin cannot be set properly

[hunterhector]

Describe the bug
The group plugin is activated only when the "center-bottom" area is not set (undefined). This behavior is a bit confusing

https://github.com/asyml/stave/blob/master/src/nlpviewer/components/TextViewer.tsx#L197

To Reproduce
Steps to reproduce the behavior:

1. Create a project by uploading any ontology
2. Try to configure the center-bottom layout, but the Group plugin won't be activated unless we delete the key.

Expected behavior

1. If the key is deleted or set to be "disable", the Group plugin should not show up.
2. It should show up when it is explicitly enabled.

Screenshots
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Environment (please complete the following information):

• OS: [e.g. iOS]
• Version [e.g. 22]
• Node Versions or other

Additional context
Add any other context about the problem here.