Understanding Swift stacktrace in Crashlytics - ios

Question

So, as a novice IOS developer I find it hard to determine what cases this exception.
Case: the IOS app has a UIScrollView that allows the user to drag down to sync with a web service - just like the build in mail app.
Once in a while a user in the wild will encounter an app crash. In crashlytics i see:
Crashed: com.apple.main-thread: EXC_BREAKPOINT 0x00000000e7ffdefe
0 myApp 0xba760 specialized WeekPlanController.scrollViewWillBeginDragging(UIScrollView) -> () (WeekPlanController.swift:428)
1 myApp 0xba4a0 specialized WeekPlanController.scrollViewWillBeginDragging(UIScrollView) -> () (WeekPlanController.swift:428)
2 myApp 0xb702c #objc WeekPlanController.scrollViewWillBeginDragging(UIScrollView) -> () (WeekPlanController.swift)
3 UIKit 0x2cee4ad3 -[UIScrollView _updatePanGesture] + 282
4 UIKit 0x2ced206b _UIGestureRecognizerSendActions + 294
5 UIKit 0x2cd810f9 -[UIGestureRecognizer _updateGestureWithEvent:buttonEvent:] + 668
6 UIKit 0x2d19af9b ___UIGestureRecognizerUpdate_block_invoke661 + 46
7 UIKit 0x2cd4a181 _UIGestureRecognizerRemoveObjectsFromArrayAndApplyBlocks + 216
8 UIKit 0x2cd4805f _UIGestureRecognizerUpdate + 2926
9 UIKit 0x2cd7f4d9 -[UIWindow _sendGesturesForEvent:] + 784
10 UIKit 0x2cd7eddd -[UIWindow sendEvent:] + 524
11 UIKit 0x2cd54fe5 -[UIApplication sendEvent:] + 196
12 UIKit 0x2cfcb8fb _UIApplicationHandleEventFromQueueEvent + 14414
13 UIKit 0x2cd539f9 _UIApplicationHandleEventQueue + 1352
14 CoreFoundation 0x296c5faf __CFRUNLOOP_IS_CALLING_OUT_TO_A_SOURCE0_PERFORM_FUNCTION__ + 14
15 CoreFoundation 0x296c53bf __CFRunLoopDoSources0 + 218
16 CoreFoundation 0x296c3a25 __CFRunLoopRun + 772
17 CoreFoundation 0x29610201 CFRunLoopRunSpecific + 476
18 CoreFoundation 0x29610013 CFRunLoopRunInMode + 106
19 GraphicsServices 0x30eef201 GSEventRunModal + 136
20 UIKit 0x2cdb4a59 UIApplicationMain + 1440
21 myApp 0x5d084 main (AppDelegate.swift:14)
22 libdyld.dylib 0x37f3daaf start + 2
This is the code from the controller:
func scrollViewWillBeginDragging(scrollView: UIScrollView){
if Settings.isSyncActive {
if let lastSyncDate = Settings.lastSyncedDate { // ln 427
self.refreshControl.attributedTitle = NSAttributedString(string: "Last sync \(lastSyncDate.relativeTimeToString()) ago") // ln 428
} // ln 429
else{
self.refreshControl.attributedTitle = NSAttributedString(string: "")
}
}
}
Any hint or tips are appreciated