fix issue with beforeDestroy hook

[dsvs12]

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This fixes the issue when the dynamic scroller disappears out of view and the this.$el.parentNode returns nothing causing the observer to have issues, stack trace for current issue follows

[Vue warn]: Error in beforeDestroy hook: "TypeError: Failed to execute 'unobserve' on 'ResizeObserver': parameter 1 is not of type 'Element'."

found in

---> <DynamicScrollerItem>
       <RecycleScroller>
         <DynamicScroller>
           <SeatSelect> at src/components/SeatSelectComponent.vue
             <Event> at src/views/Event.vue
               <App> at src/App.vue
                 <Root>
                 ```