Enabling admin notifications causes program to crash
On Release 65, If admin notifications are enabled, the software crashes. 
If admin is disabled there are no errors. 
Changing notifyBlockFound to false does not change the behavior, neither does using/adding notifyPaymentSuccess.

   "notifications": {
        "enabled": true,
        "email": {
            "host": "smtp.gmail.com",
            "port": 587,
            "user": "email@gmail.com",
            "password": "password",
            "fromAddress": "email@gmail.com",
            "fromName": "email@gmail.com"
        },
        "admin": {
            "enabled": true,
            "emailAddress": "email@gmail.com",
            "notifyBlockFound": true
        }
    },

Error:
```
[2022-01-02 23:34:35.2453] [I] [XmlKeyManager] User profile is available. Using '/home/ubuntu/.aspnet/DataProtection-Keys' as key repository; keys will not be encrypted at rest.
[2022-01-02 23:34:35.3877] [E] [Host] BackgroundService failed System.Threading.Tasks.TaskCanceledException: A task was canceled.
   at Miningcore.Notifications.NotificationService.ExecuteAsync(CancellationToken ct) in /home/ubuntu/temp/miningcore/src/Miningcore/Notifications/NotificationService.cs:line 156
   at Microsoft.Extensions.Hosting.Internal.Host.TryExecuteBackgroundServiceAsync(BackgroundService backgroundService)    at Miningcore.Notifications.NotificationService.ExecuteAsync(CancellationToken ct) in /home/ubuntu/temp/miningcore/s                                                                                rc/Miningcore/Notifications/NotificationService.cs:line 156
   at Microsoft.Extensions.Hosting.Internal.Host.TryExecuteBackgroundServiceAsync(BackgroundService backgroundService)
[2022-01-02 23:34:35.4012] [F] [Host] The HostOptions.BackgroundServiceExceptionBehavior is configured to StopHost. A BackgroundService has thrown an unhandled exception, and the IHost instance is stopping. To avoid this behavior, config                                                                                ure this to Ignore; however the BackgroundService will not be restarted. System.Threading.Tasks.TaskCanceledException: A task was canceled.
   at Miningcore.Notifications.NotificationService.ExecuteAsync(CancellationToken ct) in /home/ubuntu/temp/miningcore/src/Miningcore/Notifications/NotificationService.cs:line 156
   at Microsoft.Extensions.Hosting.Internal.Host.TryExecuteBackgroundServiceAsync(BackgroundService backgroundService)    at Miningcore.Notifications.NotificationService.ExecuteAsync(CancellationToken ct) in /home/ubuntu/temp/miningcore/s                                                                                rc/Miningcore/Notifications/NotificationService.cs:line 156
   at Microsoft.Extensions.Hosting.Internal.Host.TryExecuteBackgroundServiceAsync(BackgroundService backgroundService)
[2022-01-02 23:34:35.4012] [I] [Lifetime] Application is shutting down...
```