NullReferenceException in a wrapper

Hi,

I'm integrating Newton physics engine inside unity, everything works perfectly simulation wise, but I'm facing a strange error happening randomly during running.

While the simulation going on it will stop and throw an exception:

 NullReferenceException: Object reference not set to an instance of an object
 NewtonNet.NewtonBody.ApplyForceAndTorque (IntPtr unused, Single timestep, Int32 threadIndex) (at Assets/Plugins/NewtonNet.cs:208)
 NewtonNet.NewtonWorld.Update (Single timestep) (at Assets/Plugins/NewtonNet.cs:109)
 NewtonNet.NewtonWorld.Iterate (Single fps, Int32 iterations) (at Assets/Plugins/NewtonNet.cs:123)
 Main.Update () (at Assets/Main.cs:146)

Newton uses a callback to apply forces and torques, and it calls it every iteration. I wrap this callback to call C# delegate function this way:

 public class NewtonBody : IDisposable
 {
     [UnmanagedFunctionPointer(CallingConvention.Cdecl)]
     delegate void ApplyForceAndTorqueCallback(IntPtr body, float timestep, int threadIndex);
 
     [DllImport(Common.NewtonDLL)]
     static extern void NewtonBodySetForceAndTorqueCallback(IntPtr bodyPtr, ApplyForceAndTorqueCallback callback);
 
     public IntPtr bodyPtr;
 
     public delegate void ForceAndTorqueHandler(NewtonBody sender, float timestep, int threadIndex);
 
 
     public event ForceAndTorqueHandler ForceAndTorqueCallback;
 
     public void ApplyForceAndTorque(IntPtr unused, float timestep, int threadIndex)
     {
         if (ForceAndTorqueCallback != null)
         {
             ForceAndTorqueCallback(this, timestep, threadIndex);
         }
     }
 
     public NewtonBody(IntPtr bodyPtr)
     {
         this.bodyPtr = bodyPtr;
 
         NewtonBodySetForceAndTorqueCallback(this.bodyPtr, ApplyForceAndTorque);
     }
 }

In the main script I declare the event method like this:

 void ApplyGravity(NewtonBody body, float timestep, int threadIndex)
 {
     body.SetForce(new [] { 0f, -9.8f * body.Mass, 0f, 0f });
 }

I kind of desperate, I tried everything but nothing works. I read around it's the GC that frees some unmanaged memory but I can't tell which??

Please help! Thanks!