Steps to reproduce:

1. Open attached project "testing_canvas"
2. Open "test" scene
3. Enter play mode
-Canvas render mode is set to Screen Space - Overlay, "camera is null" is being printed out
4. Press "Screen Space - Camera" button
-Canvas render mode is changed to Screen Space - Camera, canvas.worldCamera = Camera.main, console prints out "worldCamera is not null"
5. Press "Screen Space - Overlay" button
-Canvas render mode is changed to Screen Space - Overlay, however console prints out "worldCamera is not null"

Expected behavior: Canvas.worldCamera becomes null when canvas render mode is Screen Space - Overlay

Reproduced with: 5.3.6f1, 5.4.1p1, 5.5.0b3