Got error when creating instance of SBNotificationHub - xamarin.forms

Question

I'm trying to implement push notification in my Xamarin app on iOS. Using example from docs, I have:
if (_hub == null)
{
string connectionString = "Endpoint=<whatever>";
string hubName = "<hub name>";
_hub = new SBNotificationHub(connectionString, hubName);
}
For some reason I got an exception: "Foundation.MonoTouchException: Objective-C exception thrown. Name: NSInvalidArgumentException Reason: *** -[__NSDictionaryM setObject:forKey:]: key cannot be nil Native stack trace:"
Any ideas or direction where to look are appreciated. More about error:
1 libobjc.A.dylib 0x00000001988ea9f8 objc_exception_throw + 56
2 CoreFoundation 0x0000000199688f8c _CFArgv + 0
3 CoreFoundation 0x00000001995fe2b4 <redacted> + 904
4 TennisUmpireXamariniOS 0x0000000102355038 -[SBLocalStorage readContent] + 508
5 TennisUmpireXamariniOS 0x000000010235472c -[SBLocalStorage initWithNotificationHubPath:] + 296
6 TennisUmpireXamariniOS 0x000000010234dbe4 -[SBNotificationHub initWithConnectionString:notificationHubPath:] + 420
7 TennisUmpireXamariniOS 0x0000000104530a7c wrapper_managed_to_native_ApiDefinition_Messaging_IntPtr_objc_msgSend_IntPtr_IntPtr_intptr_intptr_intptr_intptr + 236
8 TennisUmpireXamariniOS 0x0000000104521038 WindowsAzure_Messaging_SBNotificationHub__ctor_string_string + 792
9 TennisUmpireXamariniOS 0x0000000102418a00 TennisUmpireXamarin_iOS_AppDelegate_RegisteredForRemoteNotifications_UIKit_UIApplication_Foundation_NSData + 480
10 TennisUmpireXamariniOS 0x000000010271d998 wrapper_runtime_invoke_object_runtime_invoke_dynamic_intptr_intptr_intptr_intptr + 248
11 TennisUmpireXamariniOS 0x0000000104d30e80 mono_jit_runtime_invoke + 948
12 TennisUmpireXamariniOS 0x0000000104db54e8 mono_runtime_invoke_checked + 144
13 TennisUmpireXamariniOS 0x0000000104db8d90 mono_runtime_invoke + 80
14 TennisUmpireXamariniOS 0x00000001023ccf3c _ZL31native_to_managed_trampoline_23P11objc_objectP13objc_selectorPP11_MonoMethodP13UIApplicationP6NSDataj + 680
15 TennisUmpireXamariniOS 0x00000001023ccc88 -[AppDelegate application:didRegisterForRemoteNotificationsWithDeviceToken:] + 64
16 libdispatch.dylib 0x000000019914fa38 <redacted> + 24
17 libdispatch.dylib 0x00000001991507d4 <redacted> + 16
18 libdispatch.dylib 0x00000001990fe004 <redacted> + 1068
19 CoreFoundation 0x00000001996a0ec0 <redacted> + 12
20 CoreFoundation 0x000000019969bdf8 <redacted> + 1924
21 CoreFoundation 0x000000019969b354 CFRunLoopRunSpecific + 436
22 GraphicsServices 0x000000019b89b79c GSEventRunModal + 104
23 UIKitCore 0x00000001c5897b68 UIApplicationMain + 212
24 TennisUmpireXamariniOS 0x00000001031962c8 wrapper_managed_to_native_UIKit_UIApplication_UIApplicationMain_int_string___intptr_intptr + 328
25 TennisUmpireXamariniOS 0x00000001030f723c UIKit_UIApplication_Main_string___intptr_intptr + 44
26 TennisUmpireXamariniOS 0x00000001030f71fc UIKit_UIApplication_Main_string___string_string + 172
27 TennisUmpireXamariniOS 0x00000001024178c0 TennisUmpireXamarin_iOS_Application_Main_string__ + 128
28 TennisUmpireXamariniOS 0x000000010271d998 wrapper_runtime_invoke_object_runtime_invoke_dynamic_intptr_intptr_intptr_intptr + 248
29 TennisUmpireXamariniOS 0x0000000104d30e80 mono_jit_runtime_invoke + 948
30 TennisUmpireXamariniOS 0x0000000104db54e8 mono_runtime_invoke_checked + 144
31 TennisUmpireXamariniOS 0x0000000104dbb320 mono_runtime_exec_main_checked + 120
32 TennisUmpireXamariniOS 0x0000000104d1141c mono_jit_exec + 268
33 TennisUmpireXamariniOS 0x0000000104ea94d4 xamarin_main + 2184
34 TennisUmpireXamariniOS 0x00000001024177b8 main + 96
35 libdyld.dylib 0x00000001991618e0 <redacted> + 4

Answer 1

Nobody seems to encounter this problem, but I think I figured out what was happening and want to put it in writing.
SBNotificationHub component preserves the state in the storage. When constructor is called, this state is restored. And my parameters to constructor were just fine, the first call went without any errors.
However, when I tried to call RegisterTemplate method, I passed invalid parameters, which resulted in 400 error from the notification hub. The problem is that even with this invalid parameters, the state was updated.
The next call to SBNotificationHub constructor tried to restore this inconsistent state and threw the error I posted in my question.
Now I would be stuck until app removed. This is with version https://www.nuget.org/packages/Xamarin.Azure.NotificationHubs.iOS/ (1.2.5.2). The problem is that I used package https://www.nuget.org/packages/Xamarin.Azure.NotificationHubs.iOS-updated/, which is forked version that stores state in KeyChain. As we know, even if app is uninstalled, the values in KeyChain remain, so even after I re-installed my app, I got stuck, until I actually wiped out device.
So, I switched to version 1.2.5.2 to at least be able to have clean start after uninstalling the app, and then troubleshooted what was wrong with my RegisterTemplate call.
The proper behavior, I think, would be to not update state if RegisterTemplate call wasn't successful. In addition to that, it would be nice to be able to clear the state in storage (Keychain or not) if it's corrupted. Or, why constructor even need to restore the state?