android - Why does it say "unfortunately -- has stopped"? -

i'm working on note app. pretty new , wonder why says "unfortunately -- has stopped". start activity called start , when button pressed should "pg2" activity app stopps. think abot getsharedpreferences. please help, here link: https://dl.dropboxusercontent.com/u/97063669/error.png

assuming error occurs when click of reftext, error caused when attempt string string out of shared preferences. should supply actual string, empty one, default rather null.

so line like:

final string string=sgareprefences.getstring("string", "");