How to reproduce:
1. Open the attached "1290074_repro" project
2. Open the Scene "SampleScene"
3. Enter the Play Mode
4. In the Hierarchy select the "Button" GameObject (child of the "Canvas" GameObejct)
5. In the Inspector open and then close the "Select Sprite" window by pressing the little circle next to the Source Image variable name
6. In the Game view press the button

Expected results: The button is clickable, reacts to the mouse
Actual results: The button isn't clickable, doesn't react to the mouse

Reproducible with: 2020.2.0a12, 2020.2.0b11, 2021.1.0a4
Not reproducible with: 2018.4.28f1, 2019.4.14f1, 2020.1.12f1, 2020.2.0a11

Notes:
- After exiting and re-entering the Play Mode the Graphics Raycaster works again
- Selecting the "Event System" GameObject it can be seen that the Raycast doesn't hit anything hovering with a mouse over GameObjects