Fixes microsoft#186402 (microsoft#187184)

Author: hediet

src/vs/editor/browser/widget/diffEditorWidget2/diffEditorWidget2.ts

@@ -315,6 +315,11 @@ export class DiffEditorWidget2 extends DelegatingEditor implements IDiffEditor {
 	override getModel(): IDiffEditorModel | null { return this._diffModel.get()?.model ?? null; }
 
 	override setModel(model: IDiffEditorModel | null | IDiffEditorViewModel): void {
+		if (!model && this._diffModel.get()) {
+			// Transitioning from a model to no-model
+			this._reviewPane.hide();
+		}
+
 		const vm = model ? ('model' in model) ? model : this.createViewModel(model) : undefined;
 		this._editors.original.setModel(vm ? vm.model.original : null);
 		this._editors.modified.setModel(vm ? vm.model.modified : null);

src/vs/editor/browser/widget/diffEditorWidget2/diffReview.ts

@@ -277,7 +277,7 @@ export class DiffReview2 extends Disposable {
 		}
 	}
 
-	private hide(): void {
+	public hide(): void {
 		this._isVisibleObs.set(false, undefined);
 		this._diffEditor.focus();
 		this.layout();