Restore custom editors as part of layout

Fixes an incorrect assumption that a custom editor cannot be restored
if no `WebviewPanelSerializer` is registered for its view type. (Actually,
custom editors are created and restored using a custom editor provider.)

Also, ensures that `CustomEditorWidget.modelRef` satisfies the shape for the
`CustomEditorWidget` defined in `editor.ts` and cannot return `undefined`.
(However, `CustomEditorWidget.modelRef.object` can be `undefined`
until the custom editor is resolved.)

Fixes eclipse-theia#10787

packages/monaco/src/browser/monaco-editor-service.ts

@@ -67,7 +67,7 @@ export class MonacoEditorService extends StandaloneCodeEditorService {
         let editor = MonacoEditor.getCurrent(this.editors);
         if (!editor && CustomEditorWidget.is(this.shell.activeWidget)) {
             const model = this.shell.activeWidget.modelRef.object;
-            if (model.editorTextModel instanceof MonacoEditorModel) {
+            if (model?.editorTextModel instanceof MonacoEditorModel) {
                 editor = MonacoEditor.findByDocument(this.editors, model.editorTextModel)[0];
             }
         }

packages/plugin-ext/src/hosted/browser/hosted-plugin.ts

@@ -209,7 +209,8 @@ export class HostedPluginSupport extends AbstractHostedPluginSupport<PluginManag
         this.notebookRendererMessagingService.onWillActivateRenderer(rendererId => this.activateByNotebookRenderer(rendererId));
 
         this.widgets.onDidCreateWidget(({ factoryId, widget }) => {
-            if ((factoryId === WebviewWidget.FACTORY_ID || factoryId === CustomEditorWidget.FACTORY_ID) && widget instanceof WebviewWidget) {
+            // note: state restoration of custom editors is handled in `PluginCustomEditorRegistry.init`
+            if (factoryId === WebviewWidget.FACTORY_ID && widget instanceof WebviewWidget) {
                 const storeState = widget.storeState.bind(widget);
                 const restoreState = widget.restoreState.bind(widget);
 

packages/plugin-ext/src/main/browser/custom-editors/custom-editor-widget.ts

@@ -17,30 +17,36 @@
 import { injectable, inject, postConstruct } from '@theia/core/shared/inversify';
 import URI from '@theia/core/lib/common/uri';
 import { FileOperation } from '@theia/filesystem/lib/common/files';
-import { ApplicationShell, NavigatableWidget, Saveable, SaveableSource, SaveOptions } from '@theia/core/lib/browser';
+import { ApplicationShell, DelegatingSaveable, NavigatableWidget, Saveable, SaveableSource, SaveOptions } from '@theia/core/lib/browser';
 import { SaveableService } from '@theia/core/lib/browser/saveable-service';
 import { Reference } from '@theia/core/lib/common/reference';
 import { WebviewWidget } from '../webview/webview';
 import { UndoRedoService } from '@theia/editor/lib/browser/undo-redo-service';
 import { CustomEditorModel } from './custom-editors-main';
+import { CustomEditorWidget as CustomEditorWidgetShape } from '@theia/editor/lib/browser';
 
 @injectable()
-export class CustomEditorWidget extends WebviewWidget implements SaveableSource, NavigatableWidget {
+export class CustomEditorWidget extends WebviewWidget implements CustomEditorWidgetShape, SaveableSource, NavigatableWidget {
     static override FACTORY_ID = 'plugin-custom-editor';
 
     override id: string;
     resource: URI;
 
-    protected _modelRef: Reference<CustomEditorModel>;
-    get modelRef(): Reference<CustomEditorModel> {
+    protected _modelRef: Reference<CustomEditorModel | undefined> = { object: undefined, dispose: () => { } };
+    get modelRef(): Reference<CustomEditorModel | undefined> {
         return this._modelRef;
     }
     set modelRef(modelRef: Reference<CustomEditorModel>) {
+        this._modelRef.dispose();
         this._modelRef = modelRef;
+        this.delegatingSaveable.delegate = modelRef.object;
         this.doUpdateContent();
     }
+
+    // ensures that saveable is available even if modelRef.object is undefined
+    protected readonly delegatingSaveable = new DelegatingSaveable();
     get saveable(): Saveable {
-        return this._modelRef.object;
+        return this.delegatingSaveable;
     }
 
     @inject(UndoRedoService)
@@ -72,13 +78,17 @@ export class CustomEditorWidget extends WebviewWidget implements SaveableSource,
     }
 
     async save(options?: SaveOptions): Promise<void> {
-        await this._modelRef.object.saveCustomEditor(options);
+        if (this._modelRef.object) {
+            await this._modelRef.object.saveCustomEditor(options);
+        }
     }
 
     async saveAs(source: URI, target: URI, options?: SaveOptions): Promise<void> {
-        const result = await this._modelRef.object.saveCustomEditorAs(source, target, options);
-        this.doMove(target);
-        return result;
+        if (this._modelRef.object) {
+            const result = await this._modelRef.object.saveCustomEditorAs(source, target, options);
+            this.doMove(target);
+            return result;
+        }
     }
 
     getResourceUri(): URI | undefined {