Exception when refreshing the IDE demo

[NitnerocL]

Hi,
There is a bug with the Test Dlg of the IDE demo: when refreshing the page with this dialog open, it crashes.

Repro steps

1. Go to https://node-projects.github.io/dock-spawn-ts/page/demo/ide/demo.html and refresh the layout
2. Click on this button to open "Test Dlg"

3. Refresh the page
4. Now you get a JS exception

DockGraphDeserializer.ts:114 Uncaught (in promise) TypeError: Cannot read properties of null (reading 'prepareForDocking')
at DockGraphDeserializer._buildDialogs (DockGraphDeserializer.ts:114:31)
at async DockGraphDeserializer.deserialize (DockGraphDeserializer.ts:31:25)
at async DockManager.loadState (DockManager.ts:779:30)

First investigations

I found that the serialized JSON contains element: '' for the corresponding panelState, thus failing when PanelContainer.loadFromState(...) is called from DockGraphDeserializer. I don't know if there is a bug in the serialization part, or if the demo is misconfigured though.