Fix restoring side-bar widget when it was deleted previously

We need to consult the user's widget factory, just as we do
for other cases where the dock widget is missing

src/MainWindowBase.cpp

@@ -667,7 +667,7 @@ bool MainWindowBase::deserialize(const LayoutSaver::MainWindow &mw)
         const QStringList dockWidgets = mw.dockWidgetsPerSideBar.value(loc);
         for (const QString &uniqueName : dockWidgets) {
 
-            DockWidgetBase *dw = DockRegistry::self()->dockByName(uniqueName);
+            DockWidgetBase *dw = DockRegistry::self()->dockByName(uniqueName, DockRegistry::DockByNameFlag::CreateIfNotFound);
             if (!dw) {
                 qWarning() << Q_FUNC_INFO << "Could not find dock widget" << uniqueName
                            << ". Won't restore it to sidebar";

tests/tst_docks.cpp

@@ -6952,7 +6952,7 @@ void TestDocks::tst_deleteOnClose()
         QVERIFY(Testing::waitForDeleted(dock1));
         QVERIFY(!dock1.data());
 
-        //saver.restoreLayout(saved);
+        saver.restoreLayout(saved);
     }
 #endif
 }