Cannot Hide Player

Hi, I want to hide a gameobject, to make it feel like it if it is inside a Tank, I have called, a function to do it when character rides the tank:

 player.GetComponent("ThirdPersonController").SendMessage("HidePlayer");

And Hide function it's the following:

 function HidePlayer()
 {
     GameObject.Find("rootJoint").GetComponent(SkinnedMeshRenderer).enabled = false; // stop rendering the player.
     isControllable = false;    // disable player controls.
 }

But I have this next error, and I don't know what it's missing, character and tank are in play, it's something about "rootJoint" ? :

 NullReferenceException
 ThirdPersonController.HidePlayer () (at Assets/Scripts/Player/ThirdPersonController.js:96)
 UnityEngine.Component:SendMessage(String)
 ThirdPersonController:ThirdPersonController$SendMessage$System.String(Object, Object[])
 UnityScript.Lang.UnityRuntimeServices:Invoke(Object, String, Object[], Type)
 Tank:Update() (at Assets/nuevo/Scripts/Tank.js:37)

That makes reference to this line:

 GameObject.Find("rootJoint").GetComponent(SkinnedMeshRenderer).enabled = false; // stop rendering the player.

Hope you could help me Lot of thanks in advance