NSGenericException', reason: '*** Collection <__NSArrayM: 0x12a9f7d0> was mutated while being enumerated in MAPS - iphone

Question

My stacktrace is related to maps..
NSGenericException occurs, if we try modify the array, which is being enumerated... I have taken-care about not modifying the enumerating Array.
for (int k=0;k<[[af factsArray] count];k++)
//here af is my object
{
Facts *f = [[af factsArray] objectAtIndex:k];
//here i'm removing unecessary characters from my point String
NSCharacterSet *doNotWant = [NSCharacterSet characterSetWithCharactersInString:#"POINT()"];
NSString *pointsStr = [[[f.factPosition mutableCopy] componentsSeparatedByCharactersInSet:doNotWant] componentsJoinedByString:#""];
NSArray *pieces = [[pointsStr componentsSeparatedByString:#" "] copy];
CGPoint point = CGPointMake([[pieces objectAtIndex:2] floatValue], [[pieces objectAtIndex:1] floatValue]);
NSValue *val = [NSValue valueWithCGPoint:point];
[routePointsArray addObject:val];
}
NSInteger pointsCount = routePointsArray.count;
CLLocationCoordinate2D pointsToUse[pointsCount];
for(int i = 0; i < pointsCount; i++)
{
CGPoint p = [[routePointsArray objectAtIndex:i] CGPointValue];
pointsToUse[i] = CLLocationCoordinate2DMake(p.x,p.y);
}
MKPolyline *myPolyline = [MKPolyline polylineWithCoordinates:pointsToUse count:pointsCount];
[routeOverlays addObject:myPolyline];
NSLog(#"routeOverlays count:%d",[routeOverlays count]);
//adding overlays
dispatch_async(dispatch_get_main_queue(), ^{
[self.map addOverlays:routeOverlays];
});
Can anybody tell me the problem with the code because it crashes sometimes and sometimes doesn't.
here is my stacktrace
*** Terminating app due to uncaught exception 'NSGenericException', reason: '*** Collection <__NSArrayM: 0x12a9f7d0> was mutated while being enumerated.'
*** First throw call stack:
(
0 CoreFoundation 0x01a2f5e4 __exceptionPreprocess + 180
1 libobjc.A.dylib 0x017b28b6 objc_exception_throw + 44
2 CoreFoundation 0x01abf3b5 __NSFastEnumerationMutationHandler + 165
3 VectorKit 0x04029d7e -[VKMapModel layoutScene:withContext:] + 3854
4 VectorKit 0x0401fc8e -[VKModelObject layoutSceneIfNeeded:withContext:] + 110
5 VectorKit 0x04028bff -[VKMapModel layoutSceneIfNeeded:withContext:] + 143
6 VectorKit 0x0401fd35 -[VKModelObject layoutSceneIfNeeded:withContext:] + 277
7 VectorKit 0x0403f278 -[VKWorld layoutScene:withContext:] + 56
8 VectorKit 0x0402eac6 -[VKScreenCanvas drawWithTimestamp:] + 358
9 VectorKit 0x04019c15 -[VKMapCanvas drawWithTimestamp:] + 149
10 VectorKit 0x0402e4ee -[VKScreenCanvas onTimerFired:] + 142
11 libobjc.A.dylib 0x017c481f -[NSObject performSelector:withObject:] + 70
12 VectorKit 0x041adbdc -[VGLDisplayLink _displayLinkFired:] + 60
13 QuartzCore 0x045f3fca _ZN2CA7Display15DisplayLinkItem8dispatchEv + 48
14 QuartzCore 0x045f3e86 _ZN2CA7Display11DisplayLink14dispatch_itemsEyyy + 310
15 QuartzCore 0x045f43ab _ZN2CA7Display16TimerDisplayLink8callbackEP16__CFRunLoopTimerPv + 123
16 CoreFoundation 0x019edc46 __CFRUNLOOP_IS_CALLING_OUT_TO_A_TIMER_CALLBACK_FUNCTION__ + 22
17 CoreFoundation 0x019ed62d __CFRunLoopDoTimer + 1181
18 CoreFoundation 0x019d5698 __CFRunLoopRun + 1816
19 CoreFoundation 0x019d4b33 CFRunLoopRunSpecific + 467
20 CoreFoundation 0x019d494b CFRunLoopRunInMode + 123
21 GraphicsServices 0x02c469d7 GSEventRunModal + 192
22 GraphicsServices 0x02c467fe GSEventRun + 104
23 UIKit 0x0031894b UIApplicationMain + 1225
24 SnapTraq 0x0002029d main + 141
25 libdyld.dylib 0x022c1725 start + 0
)
libc++abi.dylib: terminating with uncaught exception of type NSException

Answer 1

I would like to thank #micantox for his attempt to solve my problem...
Anyways, I have solved my own problem.
I was trying to execute the loop from different threads.. that caused the actual problem.
one thread is add the annotations using the NSArray and other thread is removing those NSArray objects. to avoid this problem i have used
#synchronized(self)
{
// your code goes here
}