fix: restoring widgets from saved state is broken since v8

In 9d999d7 waiting for sessionContext.ready was removed, this
caused _loadNotebook() not to work, because it was called before the
widget metadata was available.

This commit restores waiting for sessionContext.ready. This has
to be before _loadFromKernel(), otherwise this.kernel is not
present, causing an exeption, skipping _loadNotebook().

Fixes: jupyterlab/jupyterlab#15361

python/jupyterlab_widgets/src/manager.ts

@@ -496,6 +496,7 @@ export class WidgetManager extends LabWidgetManager {
     { loadKernel, loadNotebook } = { loadKernel: true, loadNotebook: true }
   ): Promise<void> {
     try {
+      await this.context.sessionContext.ready;
       if (loadKernel) {
         try {
           this._kernelRestoreInProgress = true;