Reproduction steps:
1. Create New Project.
2. Create Canvas.
3. Create another Canvas. (Child of previous one).
4. Create one more Canvas (Child of previous one). It should be "Canvas -> Canvas -> Canvas"
5. Remove Canvas Scaler, Graphic raycaster and Canvas components from second (the middle) Canvas.
6. Add component Canvas to second (the middle) Canvas.

Expected result: Canvas component will be added.
Actual result: Unity crashes.

Reproduced on: 5.4.3p3, 5.4.4f1, 5.5.0b9, 5.5.0p3, 5.6.0a6, 5.6.0b3.
Works fine on: 5.3.7p3, 5.4.3f1, 5.5.0b7.
Regression in: 5.4.3p3(or p1, p2), 5.5.0b9(or 0b8).

Note: Adding other components might make reproduction impossible.