"NSInternalInconsistencyException: threading violation: expected the main thread" outside of my code

Question

My iOS App is crashing with the following error:

*** Assertion failure in -[FBSSerialQueue assertOnQueue], /BuildRoot/Library/Caches/com.apple.xbs/Sources/FrontBoardServices/FrontBoard-626.4.1/FrontBoardServices/FBSSerialQueue.m:98
*** Terminating app due to uncaught exception 'NSInternalInconsistencyException', reason: 'threading violation: expected the main thread'

And this is the stacktrace:

Exception: NSInternalInconsistencyException

Reason: threading violation: expected the main thread

Stacktrace: (
0   CoreFoundation                      0x000000018d56c980 DC2C95C6-B954-39E8-86A2-5E0AF8801E87 + 1255808
1   libobjc.A.dylib                     0x000000018d285028 objc_exception_throw + 60
2   CoreFoundation                      0x000000018d4694fc DC2C95C6-B954-39E8-86A2-5E0AF8801E87 + 193788
3   Foundation                          0x000000018d8aa690 7A7A96AF-79E4-3DB1-8904-42E61CAE8999 + 571024
4   FrontBoardServices                  0x00000001927ae7ec 370F1F18-E8A9-36B5-B0EB-9735A02CE2DE + 366572
5   FrontBoardServices                  0x0000000192760460 370F1F18-E8A9-36B5-B0EB-9735A02CE2DE + 46176
6   FrontBoardServices                  0x00000001927606ec 370F1F18-E8A9-36B5-B0EB-9735A02CE2DE + 46828
7   FrontBoardServices                  0x00000001927605e4 370F1F18-E8A9-36B5-B0EB-9735A02CE2DE + 46564
8   UIKitCore                           0x00000001911e1c40 AAFEFEBE-C172-3346-8972-810EB8F2F2C6 + 5995584
9   UIKitCore                           0x00000001912d1870 AAFEFEBE-C172-3346-8972-810EB8F2F2C6 + 6977648
10  UIKitCore                           0x00000001912d2640 AAFEFEBE-C172-3346-8972-810EB8F2F2C6 + 6981184
11  CameraUI                            0x00000001afbb8514 289C074B-4FFE-3D83-927A-98B268DD914F + 652564
12  libobjc.A.dylib                     0x000000018d29f3f8 objc_release + 136
13  CameraUI                            0x00000001afc5b118 289C074B-4FFE-3D83-927A-98B268DD914F + 1319192
14  libobjc.A.dylib                     0x000000018d284130 2C18C54E-6C84-310C-851F-F9602890D908 + 20784
15  libobjc.A.dylib                     0x000000018d296f80 objc_destructInstance + 92
16  libobjc.A.dylib                     0x000000018d29de44 _objc_rootDealloc + 52
17  CameraUI                            0x00000001afc502c4 289C074B-4FFE-3D83-927A-98B268DD914F + 1274564
18  libobjc.A.dylib                     0x000000018d29f3f8 objc_release + 136
19  CoreFoundation                      0x000000018d4b243c DC2C95C6-B954-39E8-86A2-5E0AF8801E87 + 492604
20  CoreFoundation                      0x000000018d43cfc0 DC2C95C6-B954-39E8-86A2-5E0AF8801E87 + 12224
21  libobjc.A.dylib                     0x000000018d29f3f8 objc_release + 136
22  CameraUI                            0x00000001afcb3704 289C074B-4FFE-3D83-927A-98B268DD914F + 1681156
23  libobjc.A.dylib                     0x000000018d284130 2C18C54E-6C84-310C-851F-F9602890D908 + 20784
24  libobjc.A.dylib                     0x000000018d296f80 objc_destructInstance + 92
25  libobjc.A.dylib                     0x000000018d29de44 _objc_rootDealloc + 52
26  CameraUI                            0x00000001afca48ac 289C074B-4FFE-3D83-927A-98B268DD914F + 1620140
27  libobjc.A.dylib                     0x000000018d29f3f8 objc_release + 136
28  libsystem_blocks.dylib              0x000000018d18fa08 _Block_release + 168
29  libobjc.A.dylib                     0x000000018d29f3f8 objc_release + 136
30  libsystem_blocks.dylib              0x000000018d18fa08 _Block_release + 168
31  libdispatch.dylib                   0x0000000102d3b27c _dispatch_client_callout + 20
32  libdispatch.dylib                   0x0000000102d4290c _dispatch_lane_serial_drain + 720
33  libdispatch.dylib                   0x0000000102d434fc _dispatch_lane_invoke + 408
34  libdispatch.dylib                   0x0000000102d4e4dc _dispatch_workloop_worker_thread + 1344
35  libsystem_pthread.dylib             0x000000018d2776d0 _pthread_wqthread + 280
36  libsystem_pthread.dylib             0x000000018d27d9e8 start_wqthread + 8
)

Is seems to be completely outside of my code. Is there any way to debug this? Is it a bug in an iOS component?

The UIImagePickerController seems to be involved. It seems to happen shortly after picking a foto via the camera.

EDIT: Here is what the debug navigator is showing right after the crash:

And one more screenshot:

I cannot identify a single action in the app that reproduces the crash. I need to click around in a component that takes a picture from the camera and saves it into a UICollectionView. Only sometimes and eventually, the crash happens at different points.

I've verified that the code inside imagePickerController:didFinishPickingMediaWithInfo: is running on the main thread.

How can I locate the exact place in my the code where this crash is happening?

EDIT 2: Update from Apple developer technical support; they believe that it is a bug and they suggest filing a bug report.

Answer 1

In iOS14 UIImagePicker is being deprecated, so I would suggest wrapping your code in an iOS version check and using the new PHPickerViewController instead. Not the best solutions, but it's something.