Steps to reproduce:
1. Open the attached project (Crash on RectTransform delete.zip)
2. Open the Assets/Scene.unity scene
3. Select "Canvas 1" and delete components GraphicRaycaster, CanvasScaler and then Canvas
4. Select "Canvas 2" and notice that the Canvas component still says "Non-root Canvases will not be scaled." (but it is the root canvas now)
5. On "Canvas 1", try changing a value on the RectTransform, or deleting the component
6. Observe the crash

Expected result: Editor should not crash
Actual result: Editor crashes

Reproduced with: 5.4.3p3, 5.4.3p4, 5.5.0f3, 5.5.0p3, 5.6.0b2
Not reproduced with: 5.3.7p3, 5.4.2p3
Regression introduced in 5.4.3p3