Group plugin cannot be set properly

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

**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.

Start in src/nlpviewer/components/TextViewer.tsx at the referenced line and reproduce the layout configuration from the issue. Check the Group plugin behavior when the center-bottom key is deleted, set to "disable", or explicitly enabled. Done means the plugin is hidden for the first two cases and shown for the third.

Written by the indexing model from the issue text.