To reproduce:
1. Create a new project
2. Create Canvas and add a cube to it
3. Set "Render Mode" in Canvas to "Screen Space - Camera"
4. In the Canvas set "Render Camera" to "Main Camera"
5. In Main Camera set "Projection" to "Orthographic"
6. Set size of "Main Camera" to 0
7. Notice that it corrupts all children of Canvas
8. Try to CTRL-Z, it doesn't bring corrupted children back

Actual result: The Cube is no more visible even if you bring main camera size to 5
Expected result: It should become visible after Main camera set to more than 0 or by using CTRL-Z

Reproduced in: 2018.3.0a5; 2018.3.5f1; 2019.1.0b2; 2019.2.0a4
Not reproducible: 2017.4.20f1; 2018.2.20f1
Can't test on 2018.3.0a4 or lower because there is no UI choice in the Editor