PHP errors in JoinListForm.

Problem/Motivation

In src/Form/JoinListForm.php, a conditional is needed around the $edit_list definition. Without it, PHP errors result when a user does not have permission for a list.

Wrong:

          $edit_list = $form['user_mailman_register']['lists']['list' . trim($list['lid'])]['options' . trim($list['lid'])];

Right:

          if (isset($form['user_mailman_register']['lists']['list' . trim($list['lid'])]['options' . trim($list['lid'])])) {
            $edit_list = $form['user_mailman_register']['lists']['list' . trim($list['lid'])]['options' . trim($list['lid'])];
          }