Repro steps:
1. Open "JSON Asset Serialization Test" project
2. Open "Default" scene
3. In the Windows menu open "Serializable Asset" window
4. Drag and drop the image from "Resources" folder to "Serializable Asset" window
5. Close the "Serializable Asset" window
6. Enter playmode
7. Observe that image appears on the plane
8. Close Unity
9. Re-open the "JSON Asset Serialization Test" project in Unity.
10. Enter playmode
11. Observe that no image is displayed on the plane
11. Open the "Serializable Asset" window
12. Observe that there is no image

-Image does not disappear after opening the project the second time and setting the image in Serializable Asset window once again.

Actual behavior: SerializedAssetLoader.LoadAsset() fails to load serialized asset after reopening Unity for the first time.
Expected behavior: Image set in Serializable Asset window stays when reopening the project for the first time.

Reproduced on: 5.3.6p2, 5.4.0f3, 5.5.0a5

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Solution: UnityEngine.JsonUtility cannot be used to serialize object references in a stable way. As a workaround (only for Editor) replace UnityEngine.JsonUtility with UnityEditor.EditorJsonUtility.