Why do I get a Null Reference Exception assigning delegate methods to the event handler Button.OnClick?

Question

This question is different from the marked duplicate as it does not go into detail about this specific example of delegates.

I would like to neatly add a delegate method to my button on click events. This is done using C# in Xamarin. When running the app in my emulator I am getting the below unhandled exception.

System.NullReferenceException: Object reference not set to an instance of an object.

Below is the code I am using

using System;
using Android.App;
using Android.Widget;
using Android.OS;

namespace AudioTour
{
    [Activity(Label = "AudioTour", MainLauncher = true, Icon = "@drawable/icon")]
    public class MainActivity : Activity
    {
        protected override void OnCreate(Bundle bundle)
        {
            base.OnCreate(bundle);
            // Find Buttons
            Button startButton = FindViewById<Button>(Resource.Id.startAudio);
            Button stopButton = FindViewById<Button>(Resource.Id.stopAudio);
            // Set Screen
            SetContentView (Resource.Layout.Main);
            // Assign Delegate Methods to Buttons
            startButton.Click += StartAudio;
            stopButton.Click += StopAudio;  
        }
        // Start Audio Method
        void StartAudio(object sender, EventArgs ea)
        {
            // Do Code
            Console.WriteLine("Starting Audio");
        }
        // Stop Audio Method
        void StopAudio(object sender, EventArgs ea)
        {
            // Do Code
            Console.WriteLine("Stopping Audio");
        }
    }
}

Thanks