Unknown problem with MissingReferenceException

Ok, so my code is only this:

 #pragma strict
 var speed : float;
 var rb : Rigidbody2D;
 
 function Start () {
     rb = GetComponent(Rigidbody2D);
 }
 
 function Update () {
     rb.velocity.y = Input.GetAxis("Vertical") * speed;
 }

My scene has only one sprite(with rigidbody2D) & the camera.

This is the error I get. What's going on?

 MissingReferenceException: The object of type 'Object' has been destroyed but you are still trying to access it.
 Your script should either check if it is null or you should not destroy the object.
 UnityEditor.Editor.IsEnabled () (at C:/buildslave/unity/build/Editor/Mono/Inspector/Editor.cs:589)
 UnityEditor.InspectorWindow.DrawEditor (UnityEditor.Editor editor, Int32 editorIndex, Boolean rebuildOptimizedGUIBlock, System.Boolean& showImportedObjectBarNext, UnityEngine.Rect& importedObjectBarRect) (at C:/buildslave/unity/build/Editor/Mono/Inspector/InspectorWindow.cs:1151)
 UnityEditor.InspectorWindow.DrawEditors (UnityEditor.Editor[] editors) (at C:/buildslave/unity/build/Editor/Mono/Inspector/InspectorWindow.cs:1028)
 UnityEditor.InspectorWindow.OnGUI () (at C:/buildslave/unity/build/Editor/Mono/Inspector/InspectorWindow.cs:352)
 System.Reflection.MonoMethod.Invoke (System.Object obj, BindingFlags invokeAttr, System.Reflection.Binder binder, System.Object[] parameters, System.Globalization.CultureInfo culture) (at /Users/builduser/buildslave/mono/build/mcs/class/corlib/System.Reflection/MonoMethod.cs:222)