Destroyed gameobject become Null ?

Hi, I have an issue, I destroyed my gameObject "ColToMove" but "if (ColToMove!=null)" is still true. I also have an error console saying that "ColToMove.transform.position" has been destroyed and that I should check if it is not null or not destroy it.

This function ReplaceCube() is called in a OnCollisionExit2D().

Is someone can explain me why "if (ColToMove!=null)" is true ?

thanks

 function ReplaceCube(col:Collision2D) {
     RandomHightDown=Random.Range(1,3);
     
     if (col.gameObject.tag != "StartCube") {
         if (ColToMove!=null) {
             if (RandomHightDown==1) {
                 ColToMove.transform.position = PreviousPosition + CubeEspaceHight;
                 PreviousPosition += CubeEspaceHight;
             }
             else if (RandomHightDown==2){
                 ColToMove.transform.position = PreviousPosition + CubeEspaceDown;
                 PreviousPosition += CubeEspaceDown;
             }
         }
         ColToMove=col;
     }
 }