NullReferenceException creating a WP8.1 Plugin

Hi,

I'm trying to set up a plugin for WP8.1 but I'm having some issues.
What I'm trying to do is creating an event from Visual Studio side this way:
public class MyPlugin {
public delegate void myDelegate(string message);
public static event myDelegate OnMyEvent;
[...]

myFunction() {
OnMyEvent(someString);
}
}

And then, use it from Unity side:
void OnEnable() {
PluginNamespace.MyPlugin.OnMyEvent += MyUnityFunction;
}
void OnDisable() {...

NullReferenceException creating a WP8.1 Plugin