Could not resolve type with token 01000012

I've been developing my app now for at least 5 months. I've been wanting to try the xamarin previewer so i updates visual studio xamarin forms and then i updated my nuget xamarin forms for my portable, ios and android.

now when i try to debug the application on the device i get

System.TypeLoadException: Could not resolve type with token 01000012

on this line

global::Xamarin.Forms.Forms.Init(this, bundle);

which i find very odd

origionally my xamarin forms nuget version was 2.3.3.168

so i thought id update to the stable version which was the same version number 2.3.3.168 but it had "stable" beside it

i've downgraded again but still get the same error.

any thoughts

thanks