每个中的getBoundingClientRect：undefined不是函数

So I am trying to call some functions when fullscreen sections are in the viewport. Let's say I have 7 sections, then I want something to happen when a certain section is inside the viewport (I have a function that snaps the sections into the viewport so there can never be multiple sections in the viewport, but I am trying to find out which section is visible in the viewport). So I am trying to call some functions when full