Backend Completion Handler crashes app if completed after PushViewController - ios

Question

What I have
I don't understand the nature of the problem.
I have 2 View Controllers :
1) FeedViewController ,which shows events in tableview
2) EventViewController, pushed when you press on event.
When Feed Loads, it start asynchronous loadings of all images of all events.
It's done for each event by following function:
EventsManager().loadProfilePictureById(event.profilePictureID as String, currentProgress: event.profilePictureProgress, completionHandler: {
(progress:Double, image:UIImage!, error:NSError!) -> Void in
event.profilePictureProgress = progress
if image != nil {
event.profilePicture = image
}
if (error == nil){
if (self.tableView.headerViewForSection(index) != nil){
var header:eventHeaderView = self.tableView.headerViewForSection(index) as! eventHeaderView
header.updateProfilePicture(
self.eventsManager.events[index].profilePictureID as String,
progress: self.eventsManager.events[index].profilePictureProgress,
image: self.eventsManager.events[index].profilePicture)
}
}else{
println("Error:" + error.description)
}
})
Here is how I push EventViewController:
func PushEventViewController(sender:UITapGestureRecognizer)->Void{
let ViewSender = sender.view!
let selectedRow = ViewSender.tag
//let Cell:HomeEventTableViewCell = TimelineEventTable.cellForRowAtIndexPath(SelectedIndexPath) as HomeEventTableViewCell
dispatch_async(dispatch_get_main_queue(), {
() -> Void in
let VC:EventViewController = self.storyboard?.instantiateViewControllerWithIdentifier("EventViewController") as! EventViewController
VC.event = self.eventsManager.events[selectedRow]
self.navigationController?.pushViewController(VC, animated: true)
})
}
Problem
The problem is that if I press on event and push EventViewController before image is downloaded (completion handlers are still getting called) it crashes app.
Assumptions
I struggled with this for days and couldn't find a solution,
but my assumptions are that completion handler called after
Crash happens when it tries to execute following line after EventViewController was pushed:
event.profilePictureProgress = progress
if image != nil {
event.profilePicture = image
}
I assume when new view controller is pushed , event object which is used in completion handler is being deallocated

Answer 1

Found out where the problem was, the issue was that variable event.profilePictureProgress was declared as dynamic var (I was going to take advantage of that to add observer to it after).