if videoChapters.isEmpty, let url = moduleSession.videoChaptersURL {
    _ = fileCacheVideoChapters.download(url)
        .then(on: .global()) { url -> Promise<[Chapter]> in
            return Chapter.readChapters(fromFile: url, ofContentId: moduleSessionID)
        }
        .done { [weak self] chapters in
            let realm = try Realm()
            if let moduleSession = realm.object(ofType: sessionType.objectType, forPrimaryKey: moduleSessionID) as? ModuleSession {
                try realm.write {
                    realm.add(chapters, update: .all)
                    moduleSession.videoChapters.append(objectsIn: chapters)
                }
            }
            self?.videoChapters = chapters.safeCopy()
            self?.sendEvent(.newData)
        }
}

Crashed: com.apple.main-thread
0  libsystem_kernel.dylib         0x1c96457b0 __pthread_kill + 8
1  libsystem_pthread.dylib        0x1e5dc09c0 pthread_kill + 212
2  libsystem_c.dylib              0x1a64bfa44 abort + 100
3  Realm                          0x101e29b70 realm::util::terminate(char const*, char const*, long) + 10
4  Realm                          0x101e29e70 realm::util::terminate_internal(std::__1::basic_stringstream<char, std::__1::char_traits<char>, std::__1::allocator<char> >&) + 768
5  Realm                          0x101e29fe8 realm::util::terminate(char const*, char const*, long, std::initializer_list<realm::util::Printable>&&) + 328
6  Realm                          0x101c9a470 realm::BPlusTreeBase::create() + 256
7  Realm                          0x101cdcb1c realm::Lst<realm::ObjKey>::insert(unsigned long, realm::ObjKey) + 80
8  Realm                          0x101df6654 realm::LnkLst::insert(unsigned long, realm::ObjKey) + 2364
9  Realm                          0x101bba0dc auto realm::List::dispatch<void realm::List::insert<objc_object* const __strong&, RLMAccessorContext>(RLMAccessorContext&, unsigned long, objc_object* const __strong&, realm::CreatePolicy)::'lambda'(objc_object* const __strong&)>(objc_object* const __strong&) const + 226 (list.hpp:226)
10 Realm                          0x101bb6754 void realm::List::insert<objc_object* const __strong&, RLMAccessorContext>(RLMAccessorContext&, unsigned long, objc_object* const __strong&, realm::CreatePolicy) + 228 (list.hpp:228)
11 Realm                          0x101bb92b8 invocation function for block in RLMInsertObject(RLMManagedArray*, objc_object*, unsigned long) + 260 (RLMManagedArray.mm:260)
12 Realm                          0x101bb9300 auto translateErrors<void () block_pointer __strong&>(void () block_pointer __strong&) + 175 (RLMManagedArray.mm:175)
13 Realm                          0x101bb68e0 changeArray(RLMManagedArray*, NSKeyValueChange, unsigned long, void () block_pointer) + 192 (RLMManagedArray.mm:192)
14 Realm                          0x101bb61a4 RLMInsertObject(RLMManagedArray*, objc_object*, unsigned long) + 261 (RLMManagedArray.mm:261)
15 Realm                          0x101bb60dc -[RLMManagedArray addObject:] + 265 (RLMManagedArray.mm:265)
16 RealmSwift                     0x10221ce48 List.append<A>(objectsIn:) + 271 (List.swift:271)
17 Optimize                       0x100d1e438 partial apply for closure #1 in closure #8 in ModuleSessionViewModel.fetchRelatedContent() + 213 (ModuleSessionViewModel.swift:213)
18 Optimize                       0x100d8e934 thunk for @callee_guaranteed () -> (@error @owned Error) + 4379896116 (<compiler-generated>:4379896116)
19 Optimize                       0x100d1e468 partial apply for thunk for @callee_guaranteed () -> (@error @owned Error) + 4379436136 (<compiler-generated>:4379436136)
20 RealmSwift                     0x102233ed0 Realm.write<A>(withoutNotifying:_:) + 255 (Realm.swift:255)
21 Optimize                       0x100d1d870 closure #8 in ModuleSessionViewModel.fetchRelatedContent() + 211 (ModuleSessionViewModel.swift:211)
22 Optimize                       0x100d1e390 partial apply for closure #10 in ModuleSessionViewModel.fetchRelatedContent() + 4379435920
23 Optimize                       0x100d1e36c partial apply for closure #8 in ModuleSessionViewModel.fetchRelatedContent() + 4379435884
24 Optimize                       0x100d1e15c partial apply for thunk for @escaping @callee_guaranteed (@guaranteed [MasterClass]) -> (@error @owned Error) + 4379435356 (<compiler-generated>:4379435356)
25 Optimize                       0x100d1e710 thunk for @escaping @callee_guaranteed (@guaranteed [Chapter]) -> (@error @owned Error)partial apply + 4379436816
26 PromiseKit                     0x101b51950 partial apply for closure #1 in closure #1 in Thenable.done(on:flags:_:) + 207 (Thenable.swift:207)
27 PromiseKit                     0x101b2e494 thunk for @escaping @callee_guaranteed () -> () + 4364 (<compiler-generated>:4364)
28 libdispatch.dylib              0x19d4d22b0 _dispatch_call_block_and_release + 24
29 libdispatch.dylib              0x19d4d3298 _dispatch_client_callout + 16
30 libdispatch.dylib              0x19d481ce0 _dispatch_main_queue_callback_4CF$VARIANT$mp + 904
31 CoreFoundation                 0x19d81a2e0 __CFRUNLOOP_IS_SERVICING_THE_MAIN_DISPATCH_QUEUE__ + 12
32 CoreFoundation                 0x19d814740 __CFRunLoopRun + 2528
33 CoreFoundation                 0x19d813818 CFRunLoopRunSpecific + 572
34 GraphicsServices               0x1b3f19570 GSEventRunModal + 160
35 UIKitCore                      0x1a013f0e8 -[UIApplication _run] + 1052
36 UIKitCore                      0x1a0144664 UIApplicationMain + 164
37 Optimize                       0x100c70d98 main + 20
38 libdyld.dylib                  0x19d4f2140 start + 4