Threading and animated controller dismissal crash - ios

Question

I have been scratching my head on this problem for some time now:
The Setup
I have a situation where I have a navigation controller (A) -> controller (B).
(B) is pushed from (A) like so:
func presentPhotoController() {
let photoController = PhotoCaptureController()
let navController = UINavigationController(rootViewController: photoController)
self.present(navController, animated: true, completion: nil)
}
Controller (B) is a camera screen (features an AVCaptureSession). A button allows to go back to (A) when pressed by dismissing (B).
The Issue
When (B) calls self.dismiss(animated: true, completion: nil), everything is working fine.
When (B) calls self.dismiss(animated: false, completion: nil) instead, which I really need it to be, I am getting a nasty crash from within the dispatch queue:
override func observeValue(forKeyPath keyPath: String?, of object: Any?, change: [NSKeyValueChangeKey : Any]?, context: UnsafeMutableRawPointer?) {
if context == &sessionRunningObserveContext {
let newValue = change?[.newKey] as AnyObject?
guard let isSessionRunning = newValue?.boolValue else { return }
DispatchQueue.main.async { [unowned self] in
// Only enable the ability to change camera if the device has more than one camera.
self.cameraButton.isEnabled = isSessionRunning && (self.videoDeviceDiscoverySession.uniqueDevicePositionsCount()) > 1
self.photoButton.isEnabled = isSessionRunning
self.flashButton.isEnabled = isSessionRunning
}
}
else {
super.observeValue(forKeyPath: keyPath, of: object, change: change, context: context)
}
}
Here are the stack traces:
libswiftCore.dylib`_swift_abortRetainUnowned:
0x100dc5598 <+0>: adr x8, #571352 ; gCRAnnotations
0x100dc559c <+4>: nop
0x100dc55a0 <+8>: adr x9, #182131 ; "attempted to retain deallocated object"
0x100dc55a4 <+12>: nop
0x100dc55a8 <+16>: str x9, [x8, #8]
-> 0x100dc55ac <+20>: brk #0x1
and:
libswiftCore.dylib`swift_unknownUnownedLoadStrong:
0x100dd4b04 <+0>: stp x20, x19, [sp, #-32]!
0x100dd4b08 <+4>: stp x29, x30, [sp, #16]
0x100dd4b0c <+8>: add x29, sp, #16 ; =16
0x100dd4b10 <+12>: ldr x19, [x0]
0x100dd4b14 <+16>: cbz x19, 0x100dd4b28 ; <+36>
0x100dd4b18 <+20>: tbnz w19, #0, 0x100dd4b30 ; <+44>
0x100dd4b1c <+24>: mov x0, x19
0x100dd4b20 <+28>: bl 0x100dc555c ; swift_unownedRetainStrong
0x100dd4b24 <+32>: b 0x100dd4b40 ; <+60>
0x100dd4b28 <+36>: movz x19, #0
0x100dd4b2c <+40>: b 0x100dd4b40 ; <+60>
0x100dd4b30 <+44>: and x0, x19, #0xfffffffffffffffe
0x100dd4b34 <+48>: bl 0x100de014c ; symbol stub for: objc_loadWeakRetained
0x100dd4b38 <+52>: mov x19, x0
0x100dd4b3c <+56>: cbz x19, 0x100dd4b50 ; <+76>
0x100dd4b40 <+60>: mov x0, x19
0x100dd4b44 <+64>: ldp x29, x30, [sp, #16]
0x100dd4b48 <+68>: ldp x20, x19, [sp], #32
0x100dd4b4c <+72>: ret
0x100dd4b50 <+76>: movz x0, #0
0x100dd4b54 <+80>: bl 0x100dc5598 ; _swift_abortRetainUnowned
The Solution?
Being new to Swift, the above feel pretty cryptic to me. If I'm guessing correctly, it seems that this crash arises from trying to retain a deallocated object ? What I don't understand is why the controller dismissal animation would change anything to it.
Could someone help me trying to figure out what's going on exactly ?
Many thanks in advance.