Can't run the app on iOS 15 Beta 5 after the second time
This maybe an issue affecting all Xamarin apps, but so far I was able to get a repro on Applications using MvvmCross.
There is an issue opened in Xamarin repo: https://github.com/xamarin/xamarin-macios/issues/12427
I have decided to add it here for awareness.

You can repro this issue on 2 example projects in MvvmCross repo (iOS device with iOS15 Beta 5 was used):
https://github.com/MvvmCross/MvvmCross-Samples/tree/master/StarWarsSample
or 
https://github.com/MvvmCross/MvvmCross-Samples/tree/master/TipCalc/TipCalc.iOS

1.Start the app;
2.Force close the app;
3.Start the app;

StarWarsSample.Forms.iOS - Crashes immediately after the restart of the app.
```
2021-08-13 10:29:04.335 StarWarsSample.Forms.iOS[2635:128198] -[AppDelegate xamarinSetGCHandle:flags:]: unrecognized selector sent to instance 0x28029f420
2021-08-13 10:29:08.419 StarWarsSample.Forms.iOS[2635:128198] 
Unhandled Exception:
Foundation.MonoTouchException: Objective-C exception thrown.  Name: NSInvalidArgumentException Reason: -[AppDelegate xamarinSetGCHandle:flags:]: unrecognized selector sent to instance 0x28029f420
Native stack trace:
	0   CoreFoundation                      0x00000001821d5328 908C804A-B43C-366E-8C4F-955F2D723734 + 598824
	1   libobjc.A.dylib                     0x000000019989c748 objc_exception_throw + 56
	2   CoreFoundation                      0x00000001822a4a64 908C804A-B43C-366E-8C4F-955F2D723734 + 1448548
	3   CoreFoundation                      0x000000018216eabc 908C804A-B43C-366E-8C4F-955F2D723734 + 178876
	4   CoreFoundation                      0x000000018216dd2c _CF_forwarding_prep_0 + 92
	5   StarWarsSample.Forms.iOS            0x0000000105784f68 xamarin_set_gchandle_with_flags_safe + 112
	6   StarWarsSample.Forms.iOS            0x0000000105784f20 xamarin_set_gchandle_with_flags_safe + 40
	7   StarWarsSample.Forms.iOS            0x00000001033e2b88 StarWarsSample.Forms.iOS + 5761928
```
StarWarsSample.iOS project does not crash immediately, but on the first tap on a table cell
```
Foundation.MonoTouchException: Objective-C exception thrown.  Name: NSInvalidArgumentException Reason: +[LOTAnimationView animationNamed:inBundle:]: unrecognized selector sent to class 0x108258100
Native stack trace:
	0   CoreFoundation                      0x00000001821d5328 908C804A-B43C-366E-8C4F-955F2D723734 + 598824
	1   libobjc.A.dylib                     0x000000019989c748 objc_exception_throw + 56
	2   CoreFoundation                      0x00000001822a49ac 908C804A-B43C-366E-8C4F-955F2D723734 + 1448364
	3   CoreFoundation                      0x000000018216eabc 908C804A-B43C-366E-8C4F-955F2D723734 + 178876
	4   CoreFoundation                      0x000000018216dd2c _CF_forwarding_prep_0 + 92
	5   libLottie.iOS.dll.dylib             0x000000010821b6d0 +[LOTAnimationView animationNamed:] + 96
	6   libLottie.iOS.dll.dylib             0x000000010820a120 wrapper_managed_to_native_ApiDefinitions_Ios_Messaging_IntPtr_objc_msgSend_IntPtr_intptr_intptr_intptr + 208
	7   libLottie.iOS.dll.dylib             0x00000001081d2824 Airbnb_Lottie_LOTAnimationView_AnimationNamed_string + 324
	8   libStarWarsSample.iOS.exe.dylib     0x000000010434a8b8 StarWarsSample_iOS_Views_PlanetView_ViewDidLoad + 3896
```
Much more simple Example project is affected as well (Set Do not link to build)
https://github.com/MvvmCross/MvvmCross-Samples/tree/master/TipCalc/TipCalc.iOS

TipCalc.iOS
```
2021-08-13 10:36:16.025 TipCalc.UI.Mac[2716:130726] -[AppDelegate xamarinSetGCHandle:flags:]: unrecognized selector sent to instance 0x283608980
2021-08-13 10:36:27.292 TipCalc.UI.Mac[2716:130726] 
Unhandled Exception:
Foundation.MonoTouchException: Objective-C exception thrown.  Name: NSInvalidArgumentException Reason: -[AppDelegate xamarinSetGCHandle:flags:]: unrecognized selector sent to instance 0x283608980
Native stack trace:
	0   CoreFoundation                      0x00000001821d5328 908C804A-B43C-366E-8C4F-955F2D723734 + 598824
	1   libobjc.A.dylib                     0x000000019989c748 objc_exception_throw + 56
	2   CoreFoundation                      0x00000001822a4a64 908C804A-B43C-366E-8C4F-955F2D723734 + 1448548
	3   CoreFoundation                      0x000000018216eabc 908C804A-B43C-366E-8C4F-955F2D723734 + 178876
	4   CoreFoundation                      0x000000018216dd2c _CF_forwarding_prep_0 + 92
	5   TipCalc.UI.Mac                      0x000000010638883c xamarin_set_gchandle_with_flags_safe + 112
	6   TipCalc.UI.Mac                      0x00000001063887f4 xamarin_set_gchandle_with_flags_safe + 40
	7   TipCalc.UI.Mac                      0x00000001046a3648 TipCalc.UI.Mac + 27883080
	8   TipCalc.UI.Mac                      0x0000000103fc4380 TipCalc.UI.Mac + 20677504
```