Applications can load other applications; therefore, you can have a hierarchy of applications, similar to the hierarchy of documents within each application. Every UIComponent class has a
parentApplication read-only property that references the Application object in which the object exists. The
parentApplication property of an Application object is never itself; it is either the Application object into which it was loaded, or it is
null (for the Application object).
Walking the application chain by using the
parentApplication property is similar to walking the document chain by using the
this line of code is in a child component like titlewindow and viewstack1 is the reference of viewstack in App(parent)