Title:
com.unity.ai.generators causes "Object of type 'System.UInt64' cannot be converted to type 'System.Int32'" error on every Hierarchy drag & drop in Unity 6000.4+

Steps to reproduce:

Create a Unity 6000.4.x project
Add com.unity.2d.enhancers (version 1.0.0) to the project — this pulls in com.unity.ai.generators 1.0.0-pre.12 as a dependency
Open any scene
Try to drag & drop any GameObject in the Hierarchy window
Actual result:
ArgumentException: Object of type 'System.UInt64' cannot be converted to type 'System.Int32' is thrown on every drag attempt. Drop never completes.

Expected result:
No errors. Drag & drop works normally.

Root cause (investigated):

com.unity.ai.generators 1.0.0-pre.12 registers a Hierarchy drop handler using the old int-based API at editor startup:

File: Modules/Unity.AI.Generators.UI/Utilities/ExternalFileDragDrop.cs, line 39:

DragAndDrop.AddDropHandler(HandleDropHierarchy);
Handler signature (line 231):

static DragAndDropVisualMode HandleDropHierarchy(
int dropTargetInstanceID, // <-- old int API
HierarchyDropFlags dropMode,
Transform parentForDraggedObjects,
bool perform)
In Unity 6000.4+, HierarchyDropHandler changed its first parameter from int to EntityId (ulong). Unity invokes all registered handlers via DynamicInvoke, passing EntityId (UInt64), which fails to convert to the registered handler's int parameter.

Note: com.unity.2d.sprite (FMOD) already correctly handles this with #if UNITY_6000_4_OR_NEWER guards. com.unity.ai.generators does not.

Fix:
Apply the same #if UNITY_6000_4_OR_NEWER pattern to ExternalFileDragDrop.cs:

#if UNITY_6000_4_OR_NEWER
static DragAndDropVisualMode HandleDropHierarchy(
EntityId dropTargetInstanceID, ...)
#else
static DragAndDropVisualMode HandleDropHierarchy(
int dropTargetInstanceID, ...)
#endif
Workaround:
Remove com.unity.2d.enhancers from the project manifest if not in use.

Versions tested:

Unity: 6000.4.5f1
com.unity.ai.generators: 1.0.0-pre.12
com.unity.2d.enhancers: 1.0.0 (the package that pulls in the dependency)