InvalidOperationException when using PushAsync

Hi,

I am using Syncfusion TabView on my MainPage.

in my App starting the page like this:

MainPage = new NavigationPage(new MainPage());

then from the MainPage I am opening Signin as Popup like this:

Navigation.PushModalAsync(new Signin());

Until here all ok...

However, on the Sign in page I have a button to go to Sign up and I am trying to open the Sign up like this:

Navigation.PushAsync(new NavigationPage(new Signup()));

or like this:

Navigation.PushModalAsync(new Signup());

but keep getting this error:

Unhandled Exception:

System.InvalidOperationException: occurred