Fix NullReferenceException on navigation for UWP

[gtbuchanan]

Thanks for pushing out the UWP integration. Unfortunately, I encounter the following NullReferenceException during navigation from a Forms page on UWP:

System.NullReferenceException: Object reference not set to an instance of an object.
   at Xamarin.Forms.Platform.UWP.VisualElementRenderer`2.SetNativeControl(TNativeElement control)
   at SignaturePad.Forms.UWP.SignaturePadRenderer.OnElementChanged(ElementChangedEventArgs`1 e)
   at Xamarin.Forms.Platform.UWP.VisualElementRenderer`2.SetElement(VisualElement element)
   at Xamarin.Forms.Platform.UWP.VisualElementRenderer`2.Dispose(Boolean disposing)
   at Xamarin.Forms.Platform.UWP.VisualElementRenderer`2.Dispose()
   at Xamarin.Forms.Platform.UWP.VisualElementExtensions.Cleanup(VisualElement self)
   at Xamarin.Forms.Platform.UWP.PageRenderer.Dispose(Boolean disposing)
   at Xamarin.Forms.Platform.UWP.VisualElementRenderer`2.Dispose()
   at Xamarin.Forms.Platform.UWP.VisualElementExtensions.Cleanup(VisualElement self)
   at Xamarin.Forms.Platform.UWP.Platform.<SetCurrent>d__45.MoveNext()
--- End of stack trace from previous location where exception was thrown ---
   at System.Runtime.CompilerServices.AsyncMethodBuilderCore.<>c.<ThrowAsync>b__6_0(Object state)
   at System.Threading.WinRTSynchronizationContext.Invoker.InvokeCore()

[dnfclas]

This seems like a small (but important) contribution, so no Contribution License Agreement is required at this point. We will now review your pull request.
Thanks,
.NET Foundation Pull Request Bot