array_key_exists(): The second argument should be either an array or an object in date\date_api_ical.inc on line 587

Public Bookings should not be sending something that is not an array of values to that function, so that's the real problem and you should file a report there. But I agree I might as well do some validity testing. We can do this much simpler than your patch by just checking once at the beginning of the function whether $form_values is an array. If it isn't nothing else will work so we just return an empty value.