[BUG] Default demo conversation throws an error

[blueknightone]

Describe the bug
The default sample conversation (Assets/DialogueSystem/Demo/Dialogue/Sample.asset) throws the follwoing error:

NullReferenceException: Object reference not set to an instance of an object
lastmilegames.DialogueSystem.DialoguePlayer.GetTargetNode (lastmilegames.DialogueSystem.NodeData.NodeLinkData nodeLink) (at Assets/DialogueSystem/Scripts/DialoguePlayer.cs:48)
lastmilegames.DialogueSystem.DialoguePlayer.PlayDialogue (lastmilegames.DialogueSystem.DialogueContainer container) (at Assets/DialogueSystem/Scripts/DialoguePlayer.cs:39)
UnityEngine.Events.InvokableCall`1[T1].Invoke (T1 args0) (at <e5b75411c8cd4bfaa9e760be20206f29>:0)
UnityEngine.Events.CachedInvokableCall`1[T].Invoke (System.Object[] args) (at <e5b75411c8cd4bfaa9e760be20206f29>:0)
UnityEngine.Events.UnityEvent.Invoke () (at <e5b75411c8cd4bfaa9e760be20206f29>:0)
UnityEngine.UI.Button.Press () (at D:/Unity/2019.4.13f1/Editor/Data/Resources/PackageManager/BuiltInPackages/com.unity.ugui/Runtime/UI/Core/Button.cs:68)
UnityEngine.UI.Button.OnPointerClick (UnityEngine.EventSystems.PointerEventData eventData) (at D:/Unity/2019.4.13f1/Editor/Data/Resources/PackageManager/BuiltInPackages/com.unity.ugui/Runtime/UI/Core/Button.cs:110)
UnityEngine.EventSystems.ExecuteEvents.Execute (UnityEngine.EventSystems.IPointerClickHandler handler, UnityEngine.EventSystems.BaseEventData eventData) (at D:/Unity/2019.4.13f1/Editor/Data/Resources/PackageManager/BuiltInPackages/com.unity.ugui/Runtime/EventSystem/ExecuteEvents.cs:50)
UnityEngine.EventSystems.ExecuteEvents.Execute[T] (UnityEngine.GameObject target, UnityEngine.EventSystems.BaseEventData eventData, UnityEngine.EventSystems.ExecuteEvents+EventFunction`1[T1] functor) (at D:/Unity/2019.4.13f1/Editor/Data/Resources/PackageManager/BuiltInPackages/com.unity.ugui/Runtime/EventSystem/ExecuteEvents.cs:261)
UnityEngine.EventSystems.EventSystem:Update() (at D:/Unity/2019.4.13f1/Editor/Data/Resources/PackageManager/BuiltInPackages/com.unity.ugui/Runtime/EventSystem/EventSystem.cs:377)

To Reproduce
Steps to reproduce the behavior:

1. Import the package (or clone the project)
2. Press play
3. Press "Play Sample" button
4. See error

Expected behavior
The sample conversation (a basic tutorial) should play

Desktop (please complete the following information):

• OS: Window 10
• Unity Version 2019.4.13f1

Additional context
This happens because the conversation is empty.