After the last update of Xamarin, I could finally see the preview of the XAML page of Xamarin.Forms project. I want to share the process with you. Just follow the below steps.
Then check if you have JDK 1.8+ installed or not. If not please install X64 version.
Update your Xamarin Forms whole solution.
Open the Forms Preview window.
You should see an error. In that case please rebuild the solution and try to open different page until you see the actual preview.