Steps to reproduce:

1.  Install behavior package (com.unity.behavior)
2.  Import the Unity Behavior Example sample
3.  Select the Example Behavior Graph Blackboard asset and manually add a new element to the {{Variables}} list in the inspector.
4. Open the SampleScene and Enter Playmode
5. Exit Playmode

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Actual results: 
{code}
NullReferenceException: Object reference not set to an instance of an object.
Unity.Behavior.Blackboard.CreateMetadata ()
Unity.Behavior.Blackboard.SetVariableValue[TValue] (Unity.Behavior.GraphFramework.SerializableGUID guid, TValue value) Unity.Behavior.RuntimeBlackboardAsset.RestoreAuthoringValue ()
Unity.Behavior.RuntimeBlackboardAsset.InternalPlaymodeStateChanged (UnityEditor.PlayModeStateChange change)
UnityEditor.EditorApplication.Internal_PlayModeStateChanged (UnityEditor.PlayModeStateChange state)
UnityEngine.GUIUtility:ProcessEvent(Int32, IntPtr, Boolean&)
{code}

Expected results: 

• No error thrown

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Reproducible with versions: 
1.0.13
Not reproducible with versions: 
1.0.12 and below