I did a clean install and all seemed to work well. After a while however (due to using Safari in parallel) the error is back and Orion is no longer usable again.
I haven't seen the code, but this looks very much as if it calls a non-existant = nil-object. So the code should check for nil before proceeding:
Process: Orion [32984]
Path: /Applications/NetSoftware/Orion.app/Contents/MacOS/Orion
Identifier: com.kagi.kagimacOS
Version: 0.99 (126.4)
Code Type: X86-64 (Native)
Parent Process: ??? [1]
Responsible: Orion [32984]
User ID: 504
Date/Time: 2023-12-13 20:13:25.661 +0100
OS Version: Mac OS X 10.14.6 (18G9323)
Report Version: 12
Anonymous UUID: 7BB40B81-71F8-6303-5A21-B2CE6A6FC1E3
Sleep/Wake UUID: F14191AF-67D1-451E-8A6D-0E616BE61B93
Time Awake Since Boot: 550000 seconds
Time Since Wake: 66000 seconds
System Integrity Protection: disabled
Crashed Thread: 0 Dispatch queue: com.apple.main-thread
Exception Type: EXC_CRASH (SIGABRT)
Exception Codes: 0x0000000000000000, 0x0000000000000000
Exception Note: EXC_CORPSE_NOTIFY
Application Specific Information:
*** Terminating app due to uncaught exception 'NSOutlineView', reason: 'NSOutlineView error inserting child indexes <NSMutableIndexSet: 0x6000035e4210>[number of indexes: 1 (in 1 ranges), indexes: (1)] in parent 0x0 (which has 0 children).'
terminating with uncaught exception of type NSException
abort() called
Application Specific Backtrace 1:
0 CoreFoundation 0x00007fff2e0f1191 __exceptionPreprocess + 256
1 libobjc.A.dylib 0x00007fff57c103c6 objc_exception_throw + 48
2 AppKit 0x00007fff2bd9895b -[NSOutlineView _validateParentRowEntry:reason:indexes:] + 0
3 AppKit 0x00007fff2b91228c -[NSOutlineView _insertItemsAtIndexes:inParentRowEntry:withAnimation:] + 184
4 AppKit 0x00007fff2b8d3a18 -[NSTableView _doUpdatedWorkWithHandler:] + 99
5 AppKit 0x00007fff2b912173 -[NSOutlineView insertItemsAtIndexes:inParent:withAnimation:] + 329
6 Orion 0x0000000106e9e05d Orion + 1896541
7 Orion 0x0000000106ea08b5 Orion + 1906869
8 Orion 0x0000000107142509 Orion + 4666633
9 libdispatch.dylib 0x00007fff5938a63d _dispatch_client_callout + 8
10 libdispatch.dylib 0x00007fff5938cde6 _dispatch_continuation_pop + 414
11 libdispatch.dylib 0x00007fff5939bf42 _dispatch_source_invoke + 2056
12 libdispatch.dylib 0x00007fff5939554b _dispatch_main_queue_callback_4CF + 813
13 CoreFoundation 0x00007fff2e03b172 CFRUNLOOP_IS_SERVICING_THE_MAIN_DISPATCH_QUEUE + 9
14 CoreFoundation 0x00007fff2e03a89f __CFRunLoopRun + 2300
15 CoreFoundation 0x00007fff2e039d51 CFRunLoopRunSpecific + 459
16 HIToolbox 0x00007fff2d2ed1ab RunCurrentEventLoopInMode + 292
17 HIToolbox 0x00007fff2d2ecee5 ReceiveNextEventCommon + 603
18 HIToolbox 0x00007fff2d2ecc76 _BlockUntilNextEventMatchingListInModeWithFilter + 64
19 AppKit 0x00007fff2b684e89 _DPSNextEvent + 1135
20 AppKit 0x00007fff2b683b77 -[NSApplication(NSEvent) _nextEventMatchingEventMask:untilDate:inMode:dequeue:] + 1361
21 AppKit 0x00007fff2b67dc94 -[NSApplication run] + 699
22 AppKit 0x00007fff2b66d1c0 NSApplicationMain + 777
23 Orion 0x0000000107456019 Orion + 7893017
24 libdyld.dylib 0x00007fff593d73d5 start + 1
25 ??? 0x0000000000000001 0x0 + 1
Thread 0 Crashed:: Dispatch queue: com.apple.main-thread
0 libsystem_kernel.dylib 0x00007fff595122c2 pthread_kill + 10
1 libsystem_pthread.dylib 0x00007fff595cdbf1 pthread_kill + 284
2 libsystem_c.dylib 0x00007fff5947c6a6 abort + 127
3 libc++abi.dylib 0x00007fff5665e641 abort_message + 231
4 libc++abi.dylib 0x00007fff5665e7df default_terminate_handler() + 267
5 libobjc.A.dylib 0x00007fff57c1277b _objc_terminate() + 97
6 libc++abi.dylib 0x00007fff5666a19e std::terminate(void (*)()) + 8
7 libc++abi.dylib 0x00007fff5666a213 std::terminate() + 51
8 libdispatch.dylib 0x00007fff5938a651 _dispatch_client_callout + 28
9 libdispatch.dylib 0x00007fff5938cde6 _dispatch_continuation_pop + 414
10 libdispatch.dylib 0x00007fff5939bf42 _dispatch_source_invoke + 2056
11 libdispatch.dylib 0x00007fff5939554b _dispatch_main_queue_callback_4CF + 813
12 com.apple.CoreFoundation 0x00007fff2e03b172 CFRUNLOOP_IS_SERVICING_THE_MAIN_DISPATCH_QUEUE + 9
13 com.apple.CoreFoundation 0x00007fff2e03a89f __CFRunLoopRun + 2300
14 com.apple.CoreFoundation 0x00007fff2e039d51 CFRunLoopRunSpecific + 459
15 com.apple.HIToolbox 0x00007fff2d2ed1ab RunCurrentEventLoopInMode + 292
16 com.apple.HIToolbox 0x00007fff2d2ecee5 ReceiveNextEventCommon + 603
17 com.apple.HIToolbox 0x00007fff2d2ecc76 _BlockUntilNextEventMatchingListInModeWithFilter + 64
18 com.apple.AppKit 0x00007fff2b684e89 _DPSNextEvent + 1135
19 com.apple.AppKit 0x00007fff2b683b77 -[NSApplication(NSEvent) _nextEventMatchingEventMask:untilDate:inMode:dequeue:] + 1361
20 com.apple.AppKit 0x00007fff2b67dc94 -[NSApplication run] + 699
21 com.apple.AppKit 0x00007fff2b66d1c0 NSApplicationMain + 777
22 com.kagi.kagimacOS 0x0000000107456019 0x106ccf000 + 7893017
23 libdyld.dylib 0x00007fff593d73d5 start + 1