Cocoa archived app crashing - swift

Question

I am new in archiving app for testing/distribution. My MacOS cocoa app crash only when built as an archive.
Crashed Thread: 0 Dispatch queue: com.apple.main-thread
Exception Type: EXC_BAD_INSTRUCTION (SIGILL)
Exception Codes: 0x0000000000000001, 0x0000000000000000
Exception Note: EXC_CORPSE_NOTIFY
Termination Signal: Illegal instruction: 4
Termination Reason: Namespace SIGNAL, Code 0x4
Terminating Process: exc handler [0]
Thread 0 Crashed:: Dispatch queue: com.apple.main-thread
0 MY.APP 0x000000010100532a specialized String._CharacterView.index(after:) + 250
1 MY.APP 0x0000000100f55ef9 specialized static TextCompletionNode.hasCommonPrefix(_:with:beyond:) + 153 (AttributeManager.swift:126)
2 MY.APP 0x0000000100f571f9 specialized TextCompletionNode.add(entry:for:) + 1161
3 MY.APP 0x0000000100f584a9 specialized closure #1 in AttributeManager.init(dataSource:loadingHandler:) + 3097
4 MY.APP 0x0000000100f5439e partial apply for closure #1 in AttributeManager.init(dataSource:loadingHandler:) + 30
5 MY.APP 0x000000010104d0a3 SearchQueryCommunicator.messageHandler(_:) + 451
6 MY.APP 0x000000010104d710 protocol witness for MessageHandler.messageHandler(_:) in conformance SearchQueryCommunicator + 16
7 MY.APP 0x000000010102ede8 Socket.handle(object:) + 456
8 MY.APP 0x0000000101031928 specialized Socket.readAvailableBytes(from:) + 1096
9 MY.APP 0x0000000101031c48 specialized Socket.stream(_:handle:) + 104 (Socket.swift:77)
10 MY.APP 0x000000010102ebff #objc Socket.stream(_:handle:) + 47
11 com.apple.CoreFoundation 0x00007fff49548dc4 _signalEventSync + 228
12 com.apple.CoreFoundation 0x00007fff49563736 _cfstream_solo_signalEventSync + 246
13 com.apple.CoreFoundation 0x00007fff49546ca4 _CFStreamSignalEvent + 484
14 com.apple.CFNetwork 0x00007fff48448a62 SocketStream::dispatchSignalFromSocketCallbackUnlocked(SocketStreamSignalHolder*) + 58
15 com.apple.CFNetwork 0x00007fff4844cff9 SocketStream::socketCallback(__CFSocket*, unsigned long, __CFData const*, void const*) + 145
16 com.apple.CFNetwork 0x00007fff4844cf2e SocketStream::_SocketCallBack_stream(__CFSocket*, unsigned long, __CFData const*, void const*, void*) + 70
17 com.apple.CoreFoundation 0x00007fff49546898 __CFSocketPerformV0 + 1016
18 com.apple.CoreFoundation 0x00007fff49527a11 __CFRUNLOOP_IS_CALLING_OUT_TO_A_SOURCE0_PERFORM_FUNCTION__ + 17
19 com.apple.CoreFoundation 0x00007fff495e142c __CFRunLoopDoSource0 + 108
20 com.apple.CoreFoundation 0x00007fff4950a470 __CFRunLoopDoSources0 + 208
21 com.apple.CoreFoundation 0x00007fff495098ed __CFRunLoopRun + 1293
22 com.apple.CoreFoundation 0x00007fff49509153 CFRunLoopRunSpecific + 483
23 com.apple.HIToolbox 0x00007fff487f3d96 RunCurrentEventLoopInMode + 286
24 com.apple.HIToolbox 0x00007fff487f3b06 ReceiveNextEventCommon + 613
25 com.apple.HIToolbox 0x00007fff487f3884 _BlockUntilNextEventMatchingListInModeWithFilter + 64
26 com.apple.AppKit 0x00007fff46aa4a73 _DPSNextEvent + 2085
27 com.apple.AppKit 0x00007fff4723ae34 -[NSApplication(NSEvent) _nextEventMatchingEventMask:untilDate:inMode:dequeue:] + 3044
28 com.apple.AppKit 0x00007fff46a99885 -[NSApplication run] + 764
29 com.apple.AppKit 0x00007fff46a68a72 NSApplicationMain + 804
30 MY.APP 0x0000000100f40949 main + 9 (AppDelegate.swift:12)
31 libdyld.dylib 0x00007fff7134a015 start + 1
Here's the TextCompletionNode function:
// we assume firstString and secondString are at least longer than beyond
private static func hasCommonPrefix(_ firstString:String, with secondString:String, beyond beyondIndex:String.Index) -> String? {
assert((firstString.endIndex >= beyondIndex) && (secondString.endIndex >= beyondIndex))
var commonPrefix = String(firstString[..<beyondIndex])
guard secondString.hasPrefix(commonPrefix) else {
return nil
}
var firstIndex = beyondIndex
let firstEndIndex = firstString.endIndex
var previousCommonPrefix: String
repeat {
previousCommonPrefix = commonPrefix
if firstIndex >= firstEndIndex {
break
}
//NSLog("firstString = \"\(firstString)\", firstIndex = \(firstIndex)")
firstIndex = firstString.index(after: firstIndex) // <-- Crashing?
commonPrefix = String(firstString[..<firstIndex])
} while secondString.hasPrefix(commonPrefix)
return previousCommonPrefix
}
Everything is working well when testing the same functionality when running the app from Xcode. I've read that debugging information is removed from archived app so you can't really debug them. I've tried to add a NSLog statement to write some debugging information in the Console app, and nothing is showing. I also tried starting the app from command-line (MyApp.app/Contents/MacOS/MyApp), nothing is showing in the terminal either.
I would like to know more about the conditions before the crash. What should I do?
EDIT: I've tried to flush STDERR and even redirect STDERR into a file based on this answer from this other post: https://stackoverflow.com/a/5938021/301189 and nothing was working. Simply replacing NSLog(_:_:) with printf(_:) helped me to get some output in the terminal. I still don't know why I can't get anything from STDERR in the archived app though.
Also, the above crash does not happen when putting output some output in the terminal. Such a weird bug...