Wrapping NSArray manipulations with '@synchronized', still crash

I have following class where I have several methods that manipulate with cachedRequests array

@interface MyClass ()
- (void) sendFromCache:(CacheData*)data;
@end
@implementation MyClass NSMutableArray *cachedRequests; - (void) initialize { cachedRequests = nil; } - (void) processCache { cachedRequests = [self getCachedRequests]; } - (void) sendNext { @synchronized (self) { if (cachedRequests != nil && [cachedRequests count] == 0){ return; } CacheData *data = [cachedRequests objectAtIndex:0]; // <-- here I get Crash if (data != nil) { [cachedRequests removeObject:requestData]; } }
}
@end

Looks like when I call: CacheData *data = [cachedRequests objectAtIndex:0]; some other thread resets cachedRequests and I get this crash.

So what I did is:

 - (void) initialize { @synchronized (self){ cachedRequests = nil; } }

The question are:

• is it enough? Do I need to add @synchronized (self) for cachedRequests = [self getCachedRequests];
• is it good practice to use:@synchronized (cachedRequests) instead?

Crash details:

2 CoreFoundation 0x18595af68 -[__NSArrayM objectAtIndex:] + 240 (NSArray.m:410)
3 living 0x100c56a1c -[RequestCache sendNext] + 168