Error for switching two screens

When I switch from the current screen to another one at the first time, everything is good. But it appears a notification that is "Unfortunately, RunRunRun (my app's name) has stoppped." when I turn back to the previous screen. How can I avoid this problem?.

Use different screens wisely

If you decided to use different screens, then you should switch them correctly, else you will run out of memory after a while...      
The recommended method of switching screens in App Inventor
(Thanks Taifun)
Also see demo: multiscreen.aia

A good alternative: