Cannot assign a ScriptableObject asset as a Dictionary key in the Inspector — a newly added entry's key field stays "None"

When a serialized Dictionary<,> uses a ScriptableObject (or a user ScriptableObject subclass) as its key type, you cannot assign an asset to the key of a newly added entry via the Inspector. After adding an entry and choosing an asset in the Object Selector, the key field remains None — the selection does not stick.

Additionally, the newly added entry is transient and never persisted: if you deselect the object and select it again (or the Inspector otherwise refreshes), the added row disappears entirely.

The problem is specific to reference-type keys that are project assets (ScriptableObjects). The same drawer works correctly when the key is a GameObject (a scene reference), which makes the issue easy to miss.

Note: assigning the key via drag-and-drop from the Project window may behave differently from assigning it via the Object Selector (picker); the picker path is the reliable repro.

Repro steps:

1. Create a ScriptableObject subclass, e.g. MyScriptor : ScriptableObject, and create at least one MyScriptor asset in the Project.
2. Add a MonoBehaviour with a serialized dictionary field keyed by that type, e.g.:

public Dictionary<MyScriptor, int> Foo = new();

1. Add the MonoBehaviour to a GameObject in a scene and select it so the Inspector shows the dictionary.
2. Click the + button to add a new entry (a new row appears with an empty key field).
3. Click the key field's Object Selector (the circle/picker), and select the MyScriptor asset.
4. Deselect the GameObject in the Hierarchy, then select it again.

Expected: The key field shows the selected MyScriptor asset; the entry becomes a real dictionary entry that persists across selection changes and saves.

Actual:

• After step 5, the key field stays None; the asset is not assigned.
• After step 6, the newly added entry is gone — it was never persisted (transient row only).

Works correctly (for contrast): With a GameObject key (Dictionary<GameObject, int>), selecting a scene GameObject in the picker assigns correctly and the entry persists.

Additional observation: On some configurations, clicking + on such a dictionary can also emit a Console warning about a skipped entry / ArgumentNullException (Value cannot be null. Parameter name: key) originating from dictionary deserialization, which appears related to the same area.

Regression / Environment: Applies to the built-in Dictionary property drawer (UI Toolkit).