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