"Switch to new" shortcut is impossible to disable or change.

1- Open Keyboard settings. Go to Shortcuts and then Workspaces.
2- Click on "Switch to new" and press Backspace to disable it.

**Result:** The shortcut Super+End still goes to the new workspace even though it says is disabled.
This problem also happens if an user try to change the shortcut to another one.

I guess this is happening because the keyboard settings is not changing "org.gnome.desktop.wm.keybindings switch-to-workspace-last".