[Solved]Unknown problem with scripts

First of all I apologise for the vague title.

I'm having multiple issues with a scene. It contains a button to get back to the menu and two ScrollRect's ("Library" and "Deck")whose contents ("LibraryContent" and "DeckContent") get created dynamically with prefabs (modified selectables). I got it working somehow, using VerticalLayoutGroup components.
Yesterday I created the Menu Scenes and connected everything with buttons and when I tried it I got these issues:
1. The contents don't get populated anymore until I enter the scene a third time. But then, when I click on one of the entries, I get a missing reference exception (at first for a text gameobject until I moved the lines of code that keep it updated to another method, now I get an exception with the type 'DeckContentScript')

Spoiler: The MissingReferenceException

MissingReferenceException: The object of type 'DeckContentScript' has been destroyed but you are still trying to access it.
Your script should either check if it is null or you should not destroy the object.
DeckContentScript.UpdateContentHeight () (at Assets/Scripts/DeckContentScript.cs:69)
DeckContentScript.DeletePrefabs (System.Collections.Generic.List`1 goL) (at Assets/Scripts/DeckContentScript.cs:62)
DeckContentScript.DoReset () (at Assets/Scripts/DeckContentScript.cs:32)
EventManagerScr.TriggerChange () (at Assets/Scripts/EventManagerScr.cs:12)
LibrarySlotScript.OnPointerDown (UnityEngine.EventSystems.PointerEventData eventdata) (at Assets/Scripts/LibrarySlotScript.cs:104)
UnityEngine.EventSystems.ExecuteEvents.Execute (IPointerDownHandler handler, UnityEngine.EventSystems.BaseEventData eventData) (at C:/buildslave/unity/build/Extensions/guisystem/UnityEngine.UI/EventSystem/ExecuteEvents.cs:38)
UnityEngine.EventSystems.ExecuteEvents.Execute[IPointerDownHandler] (UnityEngine.GameObject target, UnityEngine.EventSystems.BaseEventData eventData, UnityEngine.EventSystems.EventFunction`1 functor) (at C:/buildslave/unity/build/Extensions/guisystem/UnityEngine.UI/EventSystem/ExecuteEvents.cs:269)
UnityEngine.EventSystems.EventSystem:Update()

2. This may be unrelated, but in the GameMenu Scene, to which I go when I leave this scene, a button that is supposed to enable a GUI.window (which asks for confirmation and loads the MainMenu Scene afterwards) loads this scene instead - instantly. (I made sure that the button called the right method and that the Scene name string in that method is spelled correctly)

Since the MissingReferenceException is of no use, I have absolutely no idea what could cause the issue.
Another thing: I removed the VerticalLayoutGroup components and positioned the content via script again, hoping this would solve the problem.

The Scripts:
[removed, since the issues are solved]

 

Update: 2. The scene is no longer instantly loaded, the confirmation window now shows. But no matter which choice I make, I get returned to this scene.
Whatever I do in this menu, I get thrown right into the DeckBuilder Scene. This happens both in the Editor and in the built application regardless of whether I have been there before or not.

 

The two issues were unrelated.
I solved the first one, but sadly forgot how.

The second issue was caused by the 'click through-ability' of dynamically created gui elements. I solved this with using a prefab for pop ups.