Fatal runtime error: does not have a constructor that takes one IntPtr argument

I recently upgrad to Xamarin 5.9.7 and now getting this runtime exception in my app that occurs inconsistently. I haven't been able to determine the root cause. I appreciate any help.

This exception is a common theme on other posts, but I've read through many of them and none of the suggestions have helped. The class in question, MonoTouch.Foundation.NSTimerActionDispatcher, is not directly used in any of my code.

Oct 8 21:32:30 mansk012s-iPhone MaxRAWiPhone[564] : Unhandled managed exception:
Failed to marshal the Objective-C object 0x172e4810 (type: __Xamarin_NSTimerActionDispatcher). Could not find an existing managed instance for this object, nor was it possible to create a new managed instance (because the type 'MonoTouch.Foundation.NSTimerActionDispatcher' does not have a constructor that takes one IntPtr argument). (System.Exception)
at MonoTouch.ObjCRuntime.Runtime.MissingCtor (IntPtr ptr, IntPtr klass, System.Type type, MissingCtorResolution resolution) [0x00000] in :0
at MonoTouch.ObjCRuntime.Runtime.ConstructNSObject[NSObject] (IntPtr ptr, System.Type type, MissingCtorResolution missingCtorResolution) [0x00000] in :0
at MonoTouch.ObjCRuntime.Runtime.GetNSObject (IntPtr ptr, System.Type target_type, MissingCtorResolution missingCtorResolution, Boolean evenInFinalizerQueue, System.Boolean& created) [0x00000] in :0
at MonoTouch.ObjCRuntime.Runtime.GetNSObjectWithType (IntPtr ptr, IntPtr type_ptr, System.Boolean& created) [0x00000] in :0
at MonoTouch.ObjCRuntime.Runtime.get_nsobject_with_type (IntPtr obj, IntPtr type, System.Boolean& created) [0x00000] in :0
at (wrapper native-to-managed) MonoTouch.ObjCRuntime.Runtime:get_nsobject_with_type (intptr,intptr,int&)
at (wrapper managed-to-native) MonoTouch.UIKit.UIApplication:UIApplicationMain (int,string[],intptr,intptr)
at MonoTouch.UIKit.UIApplication.Main (System.String[] args, IntPtr principal, IntPtr delegate) [0x00000] in :0
at MonoTouch.UIKit.UIApplication.Main (System.String[] args, System.String principalClassName, System.String delegateClassName) [0x00000] in :0

Thanks,

Keith