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Cannot repro, but seeing this the first time and looks like an easy fix given it's an NRE. Just installed the latest update for VS.Mac and got the build crashing on me when trying to open a browser window from within the app or navigating back (I tapped the button to open the browser which caused the app to hang, a subsequent tap on the back button then crashed the app):
Incident Identifier: 5C069703-21B9-48D7-851B-5CCA3B1532A2
CrashReporter Key: EEF07483-AFAE-4626-ACB3-717C360F57BC
Hardware Model: iPhone7,2
Process: Ridr.Client.iOS 
Version: 1.0.8 (184.108.40.206)
Code Type: ARM-64
Parent Process: ??? 
Launch Time: 2017-08-15T12:18:27Z
OS Version: iPhone OS 10.3.3 (14G60)
Report Version: 104-Xamarin
Exception Type: SIGABRT
Exception Codes: #0 at 0x1808c9014
Crashed Thread: 0
Application Specific Information:
*** Terminating app due to uncaught exception 'System.NullReferenceException', reason: 'System.NullReferenceException: Object reference not set to an instance of an object'
Xamarin Exception Stack:
at System.Threading.Tasks.TaskCompletionSource`1[TResult]..ctor () <0x101287820 + 0x00078> in <5fd075b6ff6143c68dcfdd192b8757c5#a6893f3c44e1d83a640ceeabfdbd9715>:0
at Xamarin.Forms.Platform.iOS.NavigationRenderer.GetAppearedOrDisappearedTask (Xamarin.Forms.Page page) <0x100b9d250 + 0x0004b> in <710620a3d4a94be99b29618147548b59#a6893f3c44e1d83a640ceeabfdbd9715>:0
at Xamarin.Forms.Platform.iOS.NavigationRenderer.RemoveViewControllers (System.Boolean animated) <0x100b9db04 + 0x000cb> in <710620a3d4a94be99b29618147548b59#a6893f3c44e1d83a640ceeabfdbd9715>:0
at Xamarin.Forms.Platform.iOS.NavigationRenderer.PopViewController (System.Boolean animated) <0x100b9bcf4 + 0x00017> in <710620a3d4a94be99b29618147548b59#a6893f3c44e1d83a640ceeabfdbd9715>:0
at (wrapper managed-to-native) UIKit.UIApplication:UIApplicationMain (int,string,intptr,intptr)
at UIKit.UIApplication.Main (System.String args, System.String principalClassName, System.String delegateClassName) <0x100c6e0c8 + 0x0009b> in <d5a258578a6941c88634929b51cc6ce5#a6893f3c44e1d83a640ceeabfdbd9715>:0
at Ridr.Client.iOS.Application.Main (System.String args) <0x1000efbec + 0x0001b> in <384731c5dd324c3ab6984a4e2461a936#a6893f3c44e1d83a640ceeabfdbd9715>:0
Hey Philipp, interesting that this started happening after updating Xamarin as I am not currently aware of any changes in Xamarin.iOS that could affect this. However it might still be best to clear out the bin/ and obj/ folders and do a rebuild just to be safe. Let me know if you continue seeing other different behavior (other than the crash) afterwards.
Regarding the crash itself, it seems to be the same as in bug 38731 which is fixed in the upcoming 2.4-pre1 release.
As we haven't had any further comments on this, we will close the issue for now as resolved (per the above comment). If this issue is still being experienced on 2.4.0-pre3 or later, please reopen this is with a reproduction project to look at.