I have Ubuntu 19.04. I want to change the shortcut key for Switch to next input source to Alt+Shift. I go to the Keyboard section in Settings and find the entry (under Typing). When I click on it a dialog appears, and says "Enter new shortcut..." with a keyboard picture under it. However, when I hit Alt+Shift it is dumbly not responding.
The current shortcut is in gray color as I thought it's disabled and I don't have the privilege to change it. Accidentally, I hit Alt+PrintScreen and it responded that it is assigned to another command, but for Alt+Shift it even doesn't guide me what the problem is. How can I solve this?