ArgumentNullException error is thrown on selecting scene variables instance after deleting its prefab.

Steps to repro:
1. Create a new project.
2. Hierarchy > Click on "+" > Scene Variables.
3. Create its Prefab > Delete the Prefab.
4. Hierarchy > Select Scene Variables.

Actual Result:
ArgumentNullException error is thrown in the console.

Expected Result:
No error in the console.

Reproducible in:
2021.2.0a6, 2021.1.0b9, 2021.1.0b1.

Note: Visual Scripting was introduced in 2021.1.0b1.

Environment:
Occurring on Windows 10 & Mac 10.15.

Stack trace:
ArgumentNullException: Value cannot be null.
Parameter name: object
Unity.VisualScripting.EnsureThat.IsNotNull\[T] (T value) (at library/packagecache/com.unity.visualscripting@1.5.1-pre.5/runtime/visualscripting.core/ensure/ensurethat.objects.cs:32)
Unity.VisualScripting.Metadata.StaticObject (System.Object object) (at library/packagecache/com.unity.visualscripting@1.5.1-pre.5/editor/visualscripting.core/meta/metadata.cs:1421)
Unity.VisualScripting.Metadata.MatchWithPrefab () (at library/packagecache/com.unity.visualscripting@1.5.1-pre.5/editor/visualscripting.core/meta/metadata.cs:690)
Unity.VisualScripting.ObjectMetadata..ctor (System.Object object, System.Type definedType, Unity.VisualScripting.Metadata parent) (at library/packagecache/com.unity.visualscripting@1.5.1-pre.5/editor/visualscripting.core/meta/objectmetadata.cs:17)
Unity.VisualScripting.Metadata+DigStaticObject.<.ctor>b__0_0 (Unity.VisualScripting.Metadata parent) (at library/packagecache/com.unity.visualscripting@1.5.1-pre.5/editor/visualscripting.core/meta/metadata.cs:1387)
Unity.VisualScripting.Metadata.Dig\[TSubpath,TMetadata] (TSubpath subpath, System.Func`2\[T,TResult] constructor, System.Boolean createInPrefab, Unity.VisualScripting.Metadata prefabInstance) (at library/packagecache/com.unity.visualscripting@1.5.1-pre.5/editor/visualscripting.core/meta/metadata.cs:648)
Unity.VisualScripting.Metadata.StaticObject (System.Object object, System.Type definedType) (at library/packagecache/com.unity.visualscripting@1.5.1-pre.5/editor/visualscripting.core/meta/metadata.cs:1416)
Unity.VisualScripting.Metadata.StaticObject (System.Object object) (at library/packagecache/com.unity.visualscripting@1.5.1-pre.5/editor/visualscripting.core/meta/metadata.cs:1423)
Unity.VisualScripting.LudiqRootObjectEditor+Individual.Initialize () (at library/packagecache/com.unity.visualscripting@1.5.1-pre.5/editor/visualscripting.core/inspection/root/ludiqrootobjecteditor.cs:14)
Unity.VisualScripting.IndividualEditor.Initialize (UnityEditor.SerializedObject serializedObject, UnityEditor.Editor editorParent) (at library/packagecache/com.unity.visualscripting@1.5.1-pre.5/editor/visualscripting.core/inspection/individualeditor.cs:12)
Unity.VisualScripting.OptimizedEditor`1\[TIndividual].GetIndividualDrawer (UnityEditor.SerializedObject serializedObject) (at library/packagecache/com.unity.visualscripting@1.5.1-pre.5/editor/visualscripting.core/inspection/optimizededitor.cs:16)
Unity.VisualScripting.OptimizedEditor`1\[TIndividual].OnInspectorGUI () (at library/packagecache/com.unity.visualscripting@1.5.1-pre.5/editor/visualscripting.core/inspection/optimizededitor.cs:31)
UnityEditor.UIElements.InspectorElement+<>c__DisplayClass59_0.<CreateIMGUIInspectorFromEditor>b__0 () (at <5fdea52732634d4b84dbec4613d327ea>:0)
UnityEngine.GUIUtility:ProcessEvent(Int32, IntPtr, Boolean&)