"Error occurred" dont know why?

hi

i made a program with some activity modules

every modul plays background music (looping)
i get sometimes error running my program on my samsung galaxy

the error doesnot appears regularly, when i try it fast changing between activity modules it show me the error:

"An error hasoccurred in sub: main_activity_resume(java line: 309)
java.lang.NullPointerException Continiue?"

but sometimes it works change between the modules but after some attempts the error appears.

what could it be?

thanx
ilan