UIAlertView is causing a EXC_BAD_ACCESS and app hang - objective-c

Question

Ive the following code which is working fine , but when I add UIAlertView its start giving me exc_bad_access and the app hanged ,
The main idea is to show alert when timestamp dedtected ,
any tips what im doing wrong ?
- (void) onPayload:(PayloadEvent *) event {
NSAutoreleasePool * pool = [[NSAutoreleasePool alloc] init];
NSString *header = #"[OnPayload] ";
if (event.payloadType == TYPE_IDENTIFIED) {
if ((event.contentID != -1) && (event.timeStamp == -1)) {
[mUI performSelectorOnMainThread: #selector(Trace:) withObject:[NSString stringWithFormat:#"%# StaticID detected: %x\t\tConfidence: %f\n", header,(int)event.contentID, event.confidence] waitUntilDone:NO];
}
if ((event.timeStamp != -1) && (event.contentID == -1)) {
[mUI performSelectorOnMainThread: #selector(Trace:) withObject:[NSString stringWithFormat:#"%# Timestamp detected: %f\t\tConfidence: %f\n", header, event.timeStamp, event.confidence] waitUntilDone:NO];
UIAlertView *alert = [[UIAlertView alloc] initWithTitle:#"Poll" message:#"MY alert Poll" delegate:nil cancelButtonTitle:#"OK" otherButtonTitles: nil];
[alert show];
[alert release];
}
}
else if (event.payloadType == TYPE_NOT_IDENTIFIED) {
[mUI performSelectorOnMainThread: #selector(Trace:) withObject:[NSString stringWithFormat:#"%# Content not marked\n", header] waitUntilDone:NO];
}
else if (event.payloadType == TYPE_MARKED_BUT_NOT_IDENTIFIED) {
[mUI performSelectorOnMainThread: #selector(Trace:) withObject:[NSString stringWithFormat:#"%# Content marked but not identified\n", header] waitUntilDone:NO];
}
[event release];
[pool release];
}
My error appear here
UIAlertView *alert = [[UIAlertView alloc] initWithTitle:#"Poll" message:#"MY alert Poll" delegate:nil cancelButtonTitle:#"OK" otherButtonTitles: nil];
[alert show]; <<=====
[alert release];

Answer 1

Any code that changes the UI should be called from the main thread:
dispatch_async(dispatch_get_main_queue(), ^{
UIAlertView *alert = [[UIAlertView alloc] initWithTitle:#"Poll" message:#"MY alert Poll" delegate:nil cancelButtonTitle:#"OK" otherButtonTitles: nil];
[alert show];
[alert release];
});
On the contrary to some comments, the view that is displaying the alert will retain the UIAlertView instance, so there is no need to call autorelease instead of release.

Answer 2

Considering that you are calling -performSelectorOnMainThread a few times in the same method, I imagine it's being called on a background thread. UI changes must occur on the main thread, and thus, you should be calling [alert show]; on the main thread in order to bypass the error.
[alert performSelectorOnMainThread:#selector(show) withObject:nil waitUntilDone:NO];