Error 1120 "Access of undefined property"

Hi,

I'm suddenly getting an error message on a class assignment saying "1120: Access of undefined property button_13". There's 12 identical error messages to this, except that the last part changes depending on the instance name.

I've been using AS3 code snippets and have double checked the code, nothing seems to be working.

Screenshots below. If you need anything else let me know.

Thank you so much!