Reproduction steps:
1. Open attached user project;
2. Open "Scene.unity";
3. Select "Canvas/Image" in Hierarchy and notice the "Source Image" references a Sprite called "cat"
4. Select "Assets/cat" in Project window;
5. Change "Non Power of 2" setting from "None" to "ToNearest" in Inspector;
6. Press "Apply" button in Inspector;
7. Select "Canvas/Image" in Hierarchy.

Expected: Changing the "Non Power of 2" setting for a Sprite should not cause references to break.
Actual: the "Source Image" references displays "Missing (Sprite)".

Reproduced on: 5.6.3p2, 2017.1.0p5, 2017.2.0b9, 2017.3.0a6.

By design: Sprite was not generated after changing the texture setting, so the reference is lost.