bst remove root node fix. bst specs. - #47

if (node._parent !== null) { // change - make sure parent is not null before replacing child
  if (node._left) {
    this._replaceChild(node._parent, node, node._left);
  } else if (node._right) {
    this._replaceChild(node._parent, node, node._right);
  } else {
    this._replaceChild(node._parent, node, null);
  }
}else {
  this._root = null; // if the parent is null, we are removing root, therefore assign null to root
}