Do not serialize webview if the serializer for the webview is not set

[vitaliy-guliy]

Signed-off-by: Vitaliy Guliy vgulyy@redhat.com

What it does

Disables saving of the webview state if the plugin don't register an apropriate theia.WebviewPanelSerializer

Original downstream issue eclipse-che/che#16352

How to test

Option 1:

• write a plugin, which opens a webview but don't register an appropriate theia.WebviewPanelSerializer

Option 2 - Eclipse Che specific:

• in Eclipse Che Welcome Plugin remove a line where the plugin registers WebviewPanelSerializer

Option 3:

• will think about it

Review checklist

• as an author, I have thoroughly tested my changes and carefully followed the review guidelines

Reminder for reviewers

• as a reviewer, I agree to behave in accordance with the review guidelines

[akosyakov]

Why do we need it?

An idea was that {} object means don't restore, so a widget gets disposed in restoreState.

[akosyakov]

What should I do to reproduce it on master?

[azatsarynnyy]

It would be nice to have an easy way for a reviewer to test the proposed patch.

[vitaliy-guliy]

As it appeared, I cannot reproduce the issue on vanilla Theia.

For that I prepared simple extension, that adds a webview each time on load.
Binary of built extension could be loaded here https://github.com/vitaliy-guliy/welcome-extension/releases/download/0.0.1/welcome-extension-0.0.1.vsix.

When the extension adds vscode.WebviewPanelSerializer, we have +1 welcome page on each refresh the browser.
It's expected behavior, the extension must take into account that webview is restored and don't open it again.

But without vscode.WebviewPanelSerializer, the webview should not be persisted.

Let me check it again in Che-Theia. It could be because we use specific commit to initialize che-theia, not the latest commit form master.