Here is the full debug log:
Adding item NPC's Letter
UnityEngine.Debug:Log (object)
PixelCrushers.QuestMachine.UISSupport.UISAddItemQuestAction:Execute () (at Assets/Pixel Crushers/Quest Machine/Third Party Support/Opsive UIS Support/Scripts/Actions/UISAddItemQuestAction.cs:32)
PixelCrushers.QuestMachine.QuestNode:SetState (PixelCrushers.QuestMachine.QuestNodeState,bool) (at Assets/Plugins/Pixel Crushers/Quest Machine/Scripts/Quest/Quest Node/QuestNode.cs:423)
PixelCrushers.QuestMachine.QuestNode:SetState (PixelCrushers.QuestMachine.QuestNodeState,bool) (at Assets/Plugins/Pixel Crushers/Quest Machine/Scripts/Quest/Quest Node/QuestNode.cs:445)
PixelCrushers.QuestMachine.Quest:SetState (PixelCrushers.QuestMachine.QuestState,bool) (at Assets/Plugins/Pixel Crushers/Quest Machine/Scripts/Quest/Quest.cs:886)
PixelCrushers.QuestMachine.Quest:RuntimeStartup () (at Assets/Plugins/Pixel Crushers/Quest Machine/Scripts/Quest/Quest.cs:759)
PixelCrushers.QuestMachine.QuestListContainer/<RuntimeStartupAtEndOfFrame>d__50:MoveNext () (at Assets/Plugins/Pixel Crushers/Quest Machine/Scripts/Quest MonoBehaviours/Quest List/QuestListContainer.cs:249)
UnityEngine.GUIUtility:ProcessEvent (int,intptr)