Can not read property 0 of undefined in reactjs

Question

I have an application when user has the possibility to toggle between the components.The idea of the application is next:

1. User click on add field butt

Answer 1

Edit: I think setting state interferes with formVal updating.

setTimeout(() => {
      setFieldsOnEdit(prevIndexes => [...prevIndexes, index]);
      setDefaultMode(false);
    }, 0);

Placing it in a setTimeout resolves it.