NavigationWidget: Add missing return statement.

The activateSubWidget() function recently got a return type.

Change-Id: I3fe5434bc891c31b6d8a9e07a903eb959316b0a2
Reviewed-by: Eike Ziller <eike.ziller@theqtcompany.com>

src/plugins/coreplugin/navigationwidget.cpp

@@ -288,6 +288,7 @@ QWidget *NavigationWidget::activateSubWidget(Id factoryId)
         ICore::raiseWindow(this);
         return d->m_subWidgets.first()->widget();
     }
+    return nullptr;
 }
 
 void NavigationWidget::splitSubWidget(int factoryIndex)