FIX: Exception thrown when acessing Action's bindings after changing Composite (ISXB-494) (#1966)

A simple, low-risk fix to address the immediate problem.
Also adds a new test to cover this specific scenario.

Author: timkeo

Assets/Tests/InputSystem/CoreTests_Actions.cs

@@ -5250,6 +5250,36 @@ public void Actions_CanAddBindingsToActions_ToExistingComposite()
         composite.InsertPartBinding("Negative", "<Keyboard>/leftArrow");
         composite.InsertPartBinding("Positive", "<Keyboard>/rightArrow");
 
+        ValidateCompositeBindingsOnAction(action);
+    }
+
+    [Test]
+    [Category("Actions")]
+    [Description("ISXB-494 Changing composite of action inside a map triggered exception that wasn't caught by previous test.")]
+    public void Actions_CanChangeBindingPart_ToExistingCompositeInActionMap()
+    {
+        var keyboard = InputSystem.AddDevice<Keyboard>();
+
+        var actionMap = new InputActionMap("Map");
+        var action = actionMap.AddAction("Action", InputActionType.Value, expectedControlLayout: "Axis");
+
+        action.AddCompositeBinding("Axis")
+            .With("Negative", "<Keyboard>/a")
+            .With("Positive", "<Keyboard>/d");
+
+        Assert.That(action.bindings, Has.Count.EqualTo(3));
+        Assert.That(action.controls, Is.EquivalentTo(new[] { keyboard.aKey, keyboard.dKey }));
+
+        var composite = action.ChangeCompositeBinding("Axis");
+
+        composite.InsertPartBinding("Negative", "<Keyboard>/leftArrow");
+        composite.InsertPartBinding("Positive", "<Keyboard>/rightArrow");
+
+        ValidateCompositeBindingsOnAction(action);
+    }
+
+    private void ValidateCompositeBindingsOnAction(InputAction action)
+    {
         Assert.That(action.bindings, Has.Count.EqualTo(5));
         Assert.That(action.bindings,
             Has.Exactly(1).With.Property("isComposite").EqualTo(true).And.With.Property("isPartOfComposite").EqualTo(false).And.With

Packages/com.unity.inputsystem/CHANGELOG.md

@@ -9,6 +9,7 @@ Due to package verification, the latest version below is the unpublished version
 however, it has to be formatted properly to pass verification tests.
 
 ## [Unreleased] - yyyy-mm-dd
+- Fixed ArgumentNullException thrown when accessing Action's bindings after changing Composite part. [ISXB-494](https://issuetracker.unity3d.com/product/unity/issues/guid/ISXB-494).
 
 ### Fixed
 - Fixed default scroll speed in uGUI being slower than before. [ISXB-766](https://issuetracker.unity3d.com/product/unity/issues/guid/ISXB-766)

Packages/com.unity.inputsystem/InputSystem/Actions/InputActionSetupExtensions.cs

@@ -1609,7 +1609,7 @@ public BindingSyntax InsertPartBinding(string partName, string path)
                     throw new InvalidOperationException("Binding accessor must point to composite or part binding");
 
                 AddBindingInternal(m_ActionMap,
-                    new InputBinding { path = path, isPartOfComposite = true, name = partName },
+                    new InputBinding { path = path, isPartOfComposite = true, name = partName, action = m_Action?.name },
                     m_BindingIndexInMap + 1);
 
                 return new BindingSyntax(m_ActionMap, m_BindingIndexInMap + 1, m_Action);