Hi avk_tp,
Your error lies in this line:
set properties of the top right screen corner to {activity:mission control, modifiers:{}}
The hot-corner-activity cannot have the state 'mission control' - that's only the label you see in the System Preferences GUI. The correct state to get the 'mission control' is:
set properties of the top right screen corner to {activity:all windows, modifiers:{}}
'all windows' gets you 'mission control'.
This is the code you pasted, but corrected:
set properties of the top left screen corner to {activity:none, modifiers:{}}
set properties of the top right screen corner to {activity:all windows, modifiers:{}}
set properties of the bottom left screen corner to {activity:application windows, modifiers:{}}
set properties of the bottom right screen corner to {activity:show desktop, modifiers:{}}
Enjoy!