FIX: InputActionReference when using FEPM (ISX-1968)

When Domain Reloads are disabled, InputActionReference instances continue to reference the "old" InputAction object from the previous PlayMode session.

This fix clears the InputAction reference when exiting PlayMode allowing it to be reloaded in the next session.

Author: timkeo

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

@@ -199,6 +199,24 @@ public static InputActionReference Create(InputAction action)
             return reference;
         }
 
+        /// <summary>
+        /// Clears the cached <see cref="m_Action"/> field for all current <see cref="InputActionReference"/> objects.
+        /// </summary>
+        /// <remarks>
+        /// After calling this, the next call to <see cref="action"/> will retrieve a new <see cref="InputAction"/> reference from the existing <see cref="InputActionAsset"/> just as if
+        /// using it for the first time. The serialized <see cref="m_Asset"/> and <see cref="m_ActionId"/> fields are not touched and will continue to hold their current values.
+        /// 
+        /// This method is used to clear the Action references when exiting PlayMode since those objects are no longer valid.
+        /// </remarks>
+        internal static void ResetCachedAction()
+        {
+            var allActionRefs = Resources.FindObjectsOfTypeAll(typeof(InputActionReference));
+            foreach (InputActionReference obj in allActionRefs)
+            {
+                obj.m_Action = null;
+            }
+        }
+
         [SerializeField] internal InputActionAsset m_Asset;
         // Can't serialize System.Guid and Unity's GUID is editor only so these
         // go out as strings.

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

@@ -3662,6 +3662,9 @@ internal static void OnPlayModeChange(PlayModeStateChange change)
                     // Nuke all InputActionMapStates. Releases their unmanaged memory.
                     InputActionState.DestroyAllActionMapStates();
 
+                    // Clear the Action reference from all InputActionReference objects
+                    InputActionReference.ResetCachedAction();
+
                     // Restore settings.
                     if (!string.IsNullOrEmpty(s_SystemObject.settings))
                     {