Checkpoint script problems - C#

I'm trying to write a checkpoint script, which requires use of a singleton, forcing me to work in C# instead of javascript. I know next to nothing about coding in C#, and the Unity documentation for it is frankly awful, so I'm having a hell of a time getting anything to work.

What I want to do is to save a float called checkpoint into the singleton, which can then be accessed and changed by outside javascript scripts. However, I can't get any of it working. Here's the C# code I currently have:

using UnityEngine; public class MySingleton: MonoBehaviour { private static MySingleton instance;

 public MySingleton () 
 {
     if (instance != null)
     {
         //Debug.Log("Cannot have two instances of singleton. Self destruction in 3...");
         return;
     }

     instance = this;
 }

 public static MySingleton Instance
 {
     get
     {
         if (instance == null)
         {
             new MySingleton ();
         }

         return instance;
     }
 }
 public float checkpoint = 0;
 void Update(){
 Debug.Log(checkpoint);
 }

}

However, when it's I die and the level resets, I get the following error:

ArgumentException: CompareBaseObjects  can only be called from the main thread.
Constructors and field initializers will be executed from the loading thread when loading a scene.
Don't use this function in the constructor or field initializers, instead move initialization code to the Awake or Start function.

If i remove the : Monobehaviour from the 2nd line, I don't get that error, but the Debug.Log line fails to work. No matter which way I have it though, on resetting the level, checkpoint resets to 0, defeating the point of the thing in the first place.

Please help, it's only a few days to my deadline, and I wasn't expecting to have to learn C# all of a sudden >_<.

Thanks