FIX: NullReferenceException when using Prefab Overrides window (#1567)

* Fixed InputActionDrawerBase not initializing the TreeView correctly

- This change makes sure the TreeView.serializedObject matches the property.
The value was null which was causing `OnGUI` to throw an exception.
- The change to `UpdateSerializedObjectDirtyCount` was not necessary for this
fix but it should protect against cases where the serializedObject becomes
null for some reason. The `EditorUtility.GetDirtyCount` method returns 0
when the argument is null, so this seems like the appropriate thing to do.

* CHANGE: move changelog entry into next release

Co-authored-by: Håkan Sidenvall <hakan.sidenvall@unity3d.com>
Co-authored-by: James McGill <james.mcgill@unity3d.com>

Author: 3 people

Packages/com.unity.inputsystem/CHANGELOG.md

@@ -10,6 +10,8 @@ however, it has to be formatted properly to pass verification tests.
 
 ## [Unreleased]
 
+### Fixed
+- Fixed `ArgumentNullException` when opening the Prefab Overrides window and selecting a component with an `InputAction`.
 
 ## [1.4.3] - 2022-09-23
 
@@ -36,7 +38,6 @@ however, it has to be formatted properly to pass verification tests.
 - Fixed an issue where `ReadUnprocessedValueFromState` in PoseControl always returning default values.
 - Fix Player 1's UI controls stop working after second player joins ([case ISXB-125](https://issuetracker.unity3d.com/product/unity/issues/guid/ISXB-125)))
 
-
 ## [1.4.1] - 2022-05-30
 
 ### Fixed

Packages/com.unity.inputsystem/InputSystem/Editor/AssetEditor/InputActionTreeView.cs

@@ -1391,7 +1391,7 @@ private void OnSerializedObjectModified()
 
         public void UpdateSerializedObjectDirtyCount()
         {
-            m_SerializedObjectDirtyCount = EditorUtility.GetDirtyCount(serializedObject.targetObject);
+            m_SerializedObjectDirtyCount = serializedObject != null ? EditorUtility.GetDirtyCount(serializedObject.targetObject) : 0;
         }
 
         private bool ReloadIfSerializedObjectHasBeenChanged()

Packages/com.unity.inputsystem/InputSystem/Editor/PropertyDrawers/InputActionDrawerBase.cs

@@ -42,7 +42,7 @@ private void InitTreeIfNeeded(SerializedProperty property)
             var viewData = GetOrCreateViewData(property);
             var propertyIsClone = IsPropertyAClone(property);
 
-            if (viewData.TreeView != null && !propertyIsClone)
+            if (!propertyIsClone && viewData.TreeView != null && viewData.TreeView.serializedObject == property.serializedObject)
                 return;
 
             if (propertyIsClone)