Delete unpublish cross out strike out put a line through blue-pencil ink out

remove cut out take out edit out expunge excise eradicate cancel

You are trying to select the second item of a list, which does have only 1 item

When does the error occur? Check those events then
And generally use Do it to debug your blocks, see also tip 4 here App Inventor: How to Learn | Pura Vida Apps

see also Live Development, Testing and Debugging

Taifun

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