title: React Navigation: What does goBack(null) do?
body: <p>I am using react navigation module, and using goBack() function to close current page and navigate previous page. All params and instances must be deleted while screen is navigating.  In documentation they say : 
  'Optionally provide a key, which specifies the route to go back from. By default, goBack will close the route that it is called from. If the goal is to go back anywhere, without specifying what is getting closed, call .goBack(null);</p>
  
  <p>'What does it means? What will be changed if I use goBack(null) instead of goBack()? Thanks</p>