Xamarin.forms Firebase analystics.

I have implemented Firebase analytics tool in Xamarin.forms application, and i unable to create custom event

**public partial class FeedPage : ContentPage
{
IFirebaseAnalytics eventTracker;
public FeedPage()
{
InitializeComponent();
eventTracker = DependencyService.Get();
}

    void Handle_Clicked(object sender, System.EventArgs e)  
    {  
        eventTracker.SendEvent("Click1");   // Custom event 
    }  
}**

This custom event throw the error System.NullReferenceException: 'Object reference not set to an instance of an object.'