code: @Override
    public void endVisit(FunctionInvocation node) {
      if (Objects.equals(node.getFunctionElement(), NIL_CHK_ELEM)) {
        VariableElement var = TreeUtil.getVariableElement(node.getArgument(0));
        if (var != null) {
          addSafeVar(var);
        }
      }
    }
docs: added nil_chk's.