C# What is "at null.null" in raw stack trace?

Question

Below is part of raw stack trace from my xamarin app showed on appCenter portal.

Xamarin Exception Stack:
System.ArgumentOutOfRangeException: Index was out of range. Must be non-negative and less than the size of the collection.
Parameter name: index
  at System.Collections.Generic.List`1[T].get_Item (System.Int32 index) <0x104f9a220 + 0x0006b> 17722 in <aae03b95719e488f8a163bae4d2b5dcb#ff106360ab1772e3561f3595e3937850>:0
...
  at System.Reactive.Linq.ObservableImpl.Scan`2+_[TSource,TAccumulate].OnNext (TSource value) <0x105c92c90 + 0x0003f> in <3ad0cc868be642ac8d0c22906bb12ddb#ff106360ab1772e3561f3595e3937850>:0
  at null.null
  at System.Reactive.PlatformServices.ExceptionServicesImpl.Rethrow (System.Exception exception) <0x105cc6320 + 0x00027> in <3ad0cc868be642ac8d0c22906bb12ddb#ff106360ab1772e3561f3595e3937850>:0
...
  at UIKit.UIApplication.Main (System.String[] args, System.String principalClassName, System.String delegateClassName) <0x1044ccd90 + 0x0005f> in <e76ce71449ac4d1485f78dd27d27dab7#ff106360ab1772e3561f3595e3937850>:0
  at FF.Mobile.iOS.Application.Main (System.String[] args) <0x103167440 + 0x00023> in <527adef306d34ce889ef36b6e587c8c6#ff106360ab1772e3561f3595e3937850>:0
  at null.null
  at (wrapper managed-to-native) UIKit.UIApplication.xamarin_UIApplicationMain(int,intptr,intptr,intptr,intptr&)
  at UIKit.UIApplication.UIApplicationMain (System.Int32 argc, System.String[] argv, System.IntPtr principalClassName, System.IntPtr delegateClassName) <0x1044ccc80 + 0x0004b> in <e76ce71449ac4d1485f78dd27d27dab7#ff106360ab1772e3561f3595e3937850>:0
  at UIKit.UIApplication.Main (System.String[] args, System.String principalClassName, System.String delegateClassName) <0x1044ccd90 + 0x0005f> in <e76ce71449ac4d1485f78dd27d27dab7#ff106360ab1772e3561f3595e3937850>:0
  at FF.Mobile.iOS.Application.Main (System.String[] args) <0x103167440 + 0x00023> in <527adef306d34ce889ef36b6e587c8c6#ff106360ab1772e3561f3595e3937850>:0

What does mean lines "at null.null"?