Crash on reloading a page with slide anchor

### Description

When using `data-anchor` with slides, if I reload a page with a slide anchor in the url the app crashes with error message "can't access property "item", r.activeSlide is null".

### Link to isolated reproduction with no external CSS / JS
[codesandbox link](https://codesandbox.io/p/sandbox/fullpage-slides-bug-repro-forked-vknqnz)

### Steps to reproduce it
1. Open codesandbox link.
2. Go to "Slide2" using right arrow button.
3. Notice that "#section1/Slide2" is added to the url.
4. Reload the page inside codesandbox (not codesandbox itself).
5. Observe the application crash.

### Versions
Versions can be found within the Codesandbox link provided.