I get it most frequently when using TextMeshPro buttons.

My workaround:

[InitializeOnLoadAlways]
public class DummySelectable : Selectable
{
static DummySelectable()
{
if (s_SelectableCount < 0)
{
s_SelectableCount = 0;
}
}
}

Is Accompanied by this error:

IndexOutOfRangeException: Index was outside the bounds of the array.
(wrapper stelemref) System.Object.virt_stelemref_class(intptr,object)
UnityEngine.UI.Selectable.OnDisable () (at C:/Program Files/Unity/Hub/Editor/2019.4.3f1/Editor/Data/Resources/PackageManager/BuiltInPackages/com.unity.ugui/Runtime/UI/Core/Selectable.cs:526)
UnityEngine.GameObject:SetActive(Boolean)
ActorMenu:SetBackPackInitialPositions() (at Assets/Game/SCRIPT/ActorMenu.cs:1182)
ActorMenu:OnEnable() (at Assets/Game/SCRIPT/ActorMenu.cs:787)

as well as

IndexOutOfRangeException: Index was outside the bounds of the array.
UnityEngine.UI.Selectable.OnDisable () (at C:/Program Files/Unity/Hub/Editor/2019.4.3f1/Editor/Data/Resources/PackageManager/BuiltInPackages/com.unity.ugui/Runtime/UI/Core/Selectable.cs:523)
UnityEngine.GameObject:SetActive(Boolean)
ActorMenu:SetBackPackInitialPositions() (at Assets/Game/SCRIPT/ActorMenu.cs:1182)
ActorMenu:OnEnable() (at Assets/Game/SCRIPT/ActorMenu.cs:787)

Happens when updating a script while the game is running.
Is resolved by restarting unity, but occurs again within 15-20 minutes.
Cannot be resolved without restarting the editor.

Can be avoided by not updating a script until the game is not running. However many people use live script updating, so this would ruin said feature.

me too 2020.2.1f1

2019.4.1.f1
me too