In older DayPilot versions (including 2019 versions), the event handler object (args) was not typed (type any).
The args.originalEvent holds a MouseEvent object (unmodified event object from the click handler) and "inlineEdit" property is not a valid property:
https://developer.mozilla.org/en-US/docs/Web/API/MouseEvent
I recommend reviewing your code to make sure that what you do (extend args.originalEvent with custom properties) is intentional. If so, you can cast it to "any" so you can access this property.