Steps to reproduce:

1. Create and open a behavior graph
2. Add a blackboard variable
3. Set the shared toggle to true and make sure exposed is also on
4. Add a behavior graph agent in the game object hierarchy
5. Assign the graph
6. Set the shared variable in the agent component to anything else

Actual results: The console throws an error

Expected results: No error is thrown in the console

Reproducible with versions: 1.0.9

Tested on (OS): MacOS

Notes:

• It should probably not be editable from the inspector in the first place

Stack trace
{code:java}
NullReferenceException: Object reference not set to an instance of an object
Unity.Behavior.SharedBlackboardVariable`1[DataType].SetValue (DataType newValue, System.Boolean notifyChange) (at /Users/kristinnbragi/gitrepo/behavior/Packages/com.unity.behavior/Runtime/Execution/Blackboard/SharedBlackboardVariable.cs:183)
Unity.Behavior.SharedBlackboardVariable`1[DataType].SetObjectValueWithoutNotify (System.Object newValue) (at /Users/kristinnbragi/gitrepo/behavior/Packages/com.unity.behavior/Runtime/Execution/Blackboard/SharedBlackboardVariable.cs:167)
Unity.Behavior.BehaviorGraphAgentEditor.SetBlackboardVariableValue[DataType] (Unity.Behavior.BehaviorGraphAgent agent, Unity.Behavior.BlackboardVariable refVariable, DataType newValue) (at /Users/kristinnbragi/gitrepo/behavior/Packages/com.unity.behavior/Editor/Inspectors/BehaviorGraphAgentEditor.cs:846)
Unity.Behavior.BehaviorGraphAgentEditor.UpdateValueIfChanged[DataType] (DataType currentValue, Unity.Behavior.GraphFramework.SerializableGUID varID) (at /Users/kristinnbragi/gitrepo/behavior/Packages/com.unity.behavior/Editor/Inspectors/BehaviorGraphAgentEditor.cs:701)
Unity.Behavior.BehaviorGraphAgentEditor.DrawFieldForBlackboardVariable (Unity.Behavior.BlackboardVariable variable, System.Boolean isOverride) (at /Users/kristinnbragi/gitrepo/behavior/Packages/com.unity.behavior/Editor/Inspectors/BehaviorGraphAgentEditor.cs:306)
Unity.Behavior.BehaviorGraphAgentEditor.DrawBlackboard (System.Collections.Generic.IEnumerable`1[T] variables) (at /Users/kristinnbragi/gitrepo/behavior/Packages/com.unity.behavior/Editor/Inspectors/BehaviorGraphAgentEditor.cs:288)
Unity.Behavior.BehaviorGraphAgentEditor.OnInspectorGUI () (at /Users/kristinnbragi/gitrepo/behavior/Packages/com.unity.behavior/Editor/Inspectors/BehaviorGraphAgentEditor.cs:164)
UnityEditor.UIElements.InspectorElement+<>c__DisplayClass79_0.<CreateInspectorElementUsingIMGUI>b__0 () (at /Users/bokken/build/output/unity/unity/Editor/Mono/UIElements/Inspector/InspectorElement.cs:747)
UnityEngine.GUIUtility:ProcessEvent(Int32, IntPtr, Boolean&) (at /Users/bokken/build/output/unity/unity/Modules/IMGUI/GUIUtility.cs:219)
{code}