Wait for all notebook models to load before shutting down active sessions during python upgrade (#17766)

* handle undefined model

* wait for kernel to load before restart

* comment

* await editor model resolved

* remove cast

src/sql/workbench/contrib/notebook/browser/notebook.contribution.ts

@@ -223,6 +223,9 @@ CommandsRegistry.registerCommand({
 
 		for (let editor of editors) {
 			if (editor instanceof NotebookInput) {
+				// Wait for NotebookInput.resolve() to be called to ensure the _model field in NotebookInput
+				// is not undefined when trying to get the notebookModel.
+				await editor.modelResolved;
 				let model: INotebookModel = editor.notebookModel;
 				if (model?.providerId === JUPYTER_PROVIDER_ID) {
 					let jupyterNotebookManager: IExecuteManager = model.executeManagers.find(x => x.providerId === JUPYTER_PROVIDER_ID);