[VFX] Exception when deleting the whole graph

Jira: UUM-62094

How to reproduce:
1. Create a new Unity project using 3D (URP) or 3D (HDRP) Template
2. Make sure that project has Visual Effect Graph package installed
3. Create a new VFX Asset using Minimal System
4. Open VFX asset
5. In Update Particles block add/create Set Age Attribute
6. Select entire Graph and delete it
7. Observe the Console window

Expected result: no errors are thrown when entire Graph is deleted
Actual result: "NullReferenceException" errors are thrown when entire Graph is deleted

Reproducible with: 17.0.0 (2023.3.0a7), 17.0.2 (2023.3.0b4)
Not reproducible with: 17.0.0 (2023.3.0a6)

Note: not reproducible with other blocks like Collision, Force, FlipBook

Error:

> NullReferenceException: Object reference not set to an instance of an object
UnityEditor.VFX.ReadWritableAttributeProvider.GetAvailableString (UnityEditor.VFX.VFXModel model) (at ./Library/PackageCache/com.unity.visualeffectgraph/Editor/Models/Parameters/VFXAttributeParameter.cs:42)
UnityEditor.VFX.UI.StringPropertyRM+<>c__DisplayClass2_2.<FindStringProvider>b__1 () (at ./Library/PackageCache/com.unity.visualeffectgraph/Editor/GraphView/Views/Properties/StringPropertyRM.cs:84)
VFXSlotContainerEditor.DoInspectorGUI () (at ./Library/PackageCache/com.unity.visualeffectgraph/Editor/Inspector/VFXSlotContainerEditor.cs:59)
VFXSlotContainerEditor.OnInspectorGUI () (at ./Library/PackageCache/com.unity.visualeffectgraph/Editor/Inspector/VFXSlotContainerEditor.cs:315)
VFXBlockEditor.OnInspectorGUI () (at ./Library/PackageCache/com.unity.visualeffectgraph/Editor/Inspector/VFXBlockEditor.cs:23)
UnityEditor.UIElements.InspectorElement+<>c__DisplayClass74_0.<CreateInspectorElementUsingIMGUI>b__0 () (at /Users/bokken/build/output/unity/unity/Editor/Mono/UIElements/Inspector/InspectorElement.cs:729)
UnityEngine.GUIUtility:ProcessEvent(Int32, IntPtr, Boolean&) (at /Users/bokken/build/output/unity/unity/Modules/IMGUI/GUIUtility.cs:219)

Author: julienamsellem

Packages/com.unity.visualeffectgraph/Editor/GraphView/Views/VFXView.cs

@@ -1163,10 +1163,15 @@ void Delete(string cmd, AskUser askUser)
 
                 if (shouldClearBlackboardSelection)
                 {
-                    selection.Clear();
                     blackboard.EmptySelection();
                 }
 
+                // Remove VFX items from the editor selection
+                var newSelection = Selection.objects.ToList();
+                newSelection.RemoveAll(x => x is VFXModel);
+
+                selection.Clear();
+                Selection.objects = newSelection.ToArray();
                 controller.Remove(selectionCopy.OfType<IControlledElement>().Select(t => t.controller).ToArray(), true);
             }
         }

Tests/SRPTests/Projects/VisualEffectGraph_HDRP/Assets/AllTests/Editor/Tests/VFXGUITests.cs

@@ -11,6 +11,7 @@
 using UnityEngine;
 using UnityEngine.TestTools;
 using Moq;
+using UnityEditor.Experimental.GraphView;
 using UnityEditor.Search;
 using UnityEditor.SearchService;
 using UnityEditor.VFX.UI;
@@ -945,6 +946,35 @@ public IEnumerator Check_ObjectPropertyRMTextureSearch()
             }
         }
 
+        [UnityTest, Description("Covers: UUM-61929")]
+        public IEnumerator Check_Selection_Is_Emptied_On_Delete()
+        {
+            // Prepare
+            EditorWindow.GetWindow<InspectorWindow>(); // Show the inspector because the issue was triggered by our inspector editor
+            EditorWindow.GetWindow<ProjectBrowser>(); // Show the project browser to select asset (so that the selection is not empty)
+
+            var window = CreateSimpleVFXGraph();
+            var setAgeOperatorDesc = VFXLibrary.GetBlocks().FirstOrDefault(o => o.name == "Set age");
+            var setAgeBlock = setAgeOperatorDesc.CreateInstance();
+            window.graphView.controller.contexts.Single(x => x.model is VFXBasicUpdate).model.AddChild(setAgeBlock);
+            window.graphView.controller.ApplyChanges();
+
+            window.graphView.Focus();
+            window.graphView.AddRangeToSelection(window.graphView
+                .GetAllNodes()
+                .Union(window.graphView.nodes.Where(x => x is VFXBlockUI block && block.controller.model is SetAttribute).OfType<ISelectable>())
+                .ToList());
+            Selection.Add(window.displayedResource.asset);
+            Assert.IsTrue(Selection.objects?.Length > 1);
+
+            // Act
+            window.graphView.Delete();
+            yield return null;
+
+            // Assert
+            Assert.IsTrue(Selection.objects?.Length == 1);
+        }
+
         private void CheckNumericPropertyRM<T,U>(Func<IPropertyRMProvider, NumericPropertyRM<T, U>> creator, UnityEngine.PropertyAttribute attribute, T initialValue, List<(T, T)> testCases)
         {
             // Arrange