Script Error

I have this script which I think is supposed to set the height of my terrain so I can lower it but when I click on the option which is called raise or lower terrain I get an error saying "method not found UnityEngine.Object.GetComponent". Can anyone help me out with this?

 class RaiseHeightmap extends ScriptableWizard {

 var addHeight = .1;

 static var terrain : TerrainData;

 

 @MenuItem ("Terrain/Raise or Lower Heightmap...")

 static function CreateWizard () {

     terrain = null;

     var terrainObject : Terrain = Selection.activeObject.GetComponent(Terrain) as Terrain;

     if (!terrainObject) {

         terrainObject = Terrain.activeTerrain;

     }

     if (terrainObject) {

         terrain = terrainObject.terrainData;

         var buttonText = "Apply Height";

     }

     else {

         buttonText = "Cancel";

     }

     ScriptableWizard.DisplayWizard("Raise/Lower Heightmap", RaiseHeightmap, buttonText);

 }



 function OnWizardUpdate () {

     if (!terrain) {

         helpString = "No terrain found";

         return;

     }



     addHeight = Mathf.Clamp(addHeight, -1.0, 1.0);

     helpString = (terrain.size.y*addHeight) + " meters (" + parseInt(addHeight*100.0) + "%)";

 }

 

 function OnWizardCreate () {

     if (!terrain) {

         return;

     }

     Undo.RegisterUndo(terrain, "Raise or Lower Heightmap");

     

     var heights = terrain.GetHeights(0, 0, terrain.heightmapWidth, terrain.heightmapHeight);

     for (y = 0; y < terrain.heightmapHeight; y++) {

         for (x = 0; x < terrain.heightmapWidth; x++) {

             heights[y,x] = heights[y,x] + addHeight;

         }

     }

     terrain.SetHeights(0, 0, heights);

     terrain = null;

 }

}