I found this @Anke extension very useful to close the application and leave no instance open when there are switchs between different screens »
And I found this 'Tribblehunter method' to make the switch between the screens, very functional *»
( * ) » Extracted from here » [App Inventor Tutorials and Examples: Multiple Screens | Pura Vida Apps]
To get out of the application, once its final process is completed, it is not necessary to use the [Close Application] Block (from the control drawer)... Just invoke the extension and call by [Close Screen] 
Lito
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