Select list item error: “Attempt to get item number 0” (but logic seems correct)

MIT App Inventor Help
1

Hi everyone,

I’m building a paging UI in MIT App Inventor with:

  • 8 products in a list
  • 5 buttons showing products
  • FastForward / FastBackward buttons

Logic: circular paging using modulo/remainder.

Problem:
I keep getting this runtime error:

Select list item: Attempt to get item number 0. The minimum valid item number is 1.

Even though:

  • StartIndex is initialized to 1
  • CurrentIndex is calculated using modulo
  • App mostly works visually

Refresh logic (simplified):

CurrentIndex = ((StartIndex + number − 2) mod length Products) + 1

Buttons set using ButtonList[number].

Has anyone faced this issue where modulo logic is correct but index 0 error still appears?
What could cause CurrentIndex to briefly become 0?

Thanks in advance :blush: attached is the screen shoot of relevent blocks and its aia

blocks (22)
blocks (22)2279×1028 232 KB

flung (1).aia (1.0 MB)

2

put the set button text block inside the if statement, it is probably running before Currentindex is set to length of list.

3

grafik
grafik1220×372 33.8 KB

grafik
grafik1135×329 30.3 KB

Since the forward and backward buttons are not declared in the button list, the error is inevitable.