Objective-C crash - ios

Question

I'm checking self.next_id is not null why would this line crash? I can't reproduce the issue on my test devices. I only get crash reports from Crashlytics at this line.
-(Second*)getNextSecond:(BOOL)returnSelf{
Second* retValue = nil;
if(self.next_id){
//this line !!!!
retValue = [[SecondDatabase sharedManager] getSecond:[self.next_id stringValue]];
}
if (retValue == nil && returnSelf) {
return self;
}else{
return retValue;
}
}
This is my SecondDatabase sharedManager function:
#property NSMutableDictionary* dictionary;
- (id)init {
if (self = [super init]) {
_dictionary = [#{} mutableCopy];
}
return self;
}
+ (id)sharedManager {
static SecondDatabase *sharedMyManager = nil;
static dispatch_once_t onceToken;
dispatch_once(&onceToken, ^{
sharedMyManager = [[self alloc] init];
});
return sharedMyManager;
}
This is my getSecond function:
-(Second*)getSecond:(NSString*)idString{
return [_dictionary objectForKey:idString];
}
Where should I be investigating? Would this crash if dictionary didn't have object for key so that it returns NULL?
My crash report:
Crashed: com.apple.main-thread
EXC_BAD_ACCESS KERN_INVALID_ADDRESS at 0x000007d8
Thread : Crashed: com.apple.main-thread
0 libobjc.A.dylib 0x3acd164c realizeClass(objc_class*) + 19
1 libobjc.A.dylib 0x3acd1715 realizeClass(objc_class*) + 220
2 libobjc.A.dylib 0x3acd1715 realizeClass(objc_class*) + 220
3 libobjc.A.dylib 0x3acd39ab lookUpImpOrForward + 74
4 libobjc.A.dylib 0x3acd3957 _class_lookupMethodAndLoadCache3 + 34
5 libobjc.A.dylib 0x3acd88b9 _objc_msgSend_uncached + 24
6 itsmysecond-ios 0x001287c7 -[Second getNextSecond:] (Second.m:93)
7 itsmysecond-ios 0x000fd5ff -[EBParallaxViewController adjustViewsToSeconds] (EBParallaxViewController.m:181)
8 itsmysecond-ios 0x000fd21d -[EBParallaxViewController scrollViewDidEndDecelerating:] (EBParallaxViewController.m:154)
9 UIKit 0x32ed6957 -[UIScrollView(UIScrollViewInternal) _stopScrollDecelerationNotify:] + 806
10 UIKit 0x32e0cd47 -[UIScrollView(UIScrollViewInternal) _stopScrollingNotify:pin:tramplingDragFlags:] + 466
11 UIKit 0x32e0cb6b -[UIScrollView(UIScrollViewInternal) _stopScrollingNotify:pin:] + 30
12 UIKit 0x32ed621b -[UIScrollView _smoothScrollWithUpdateTime:] + 3322
13 QuartzCore 0x32a06df3 CA::Display::DisplayLinkItem::dispatch() + 98
14 QuartzCore 0x32a06b9d CA::Display::DisplayLink::dispatch_items(unsigned long long, unsigned long long, unsigned long long) + 344
15 IOMobileFramebuffer 0x3577f75d IOMobileFramebufferVsyncNotifyFunc + 104
16 IOKit 0x31209451 IODispatchCalloutFromCFMessage + 248
17 CoreFoundation 0x304ddea9 __CFMachPortPerform + 136
18 CoreFoundation 0x304e8a67 __CFRUNLOOP_IS_CALLING_OUT_TO_A_SOURCE1_PERFORM_FUNCTION__ + 34
19 CoreFoundation 0x304e8a03 __CFRunLoopDoSource1 + 346
20 CoreFoundation 0x304e71d7 __CFRunLoopRun + 1398
21 CoreFoundation 0x30451ebf CFRunLoopRunSpecific + 522
22 CoreFoundation 0x30451ca3 CFRunLoopRunInMode + 106
23 GraphicsServices 0x35357663 GSEventRunModal + 138
24 UIKit 0x32d9e14d UIApplicationMain + 1136
25 itsmysecond-ios 0x00109707 main (main.m:14)
26 libdyld.dylib 0x3b1dbab7 start + 2