message: fix bug that prevented a line from being removed when undoing a newline
diff: @@ -176,7 +176,7 @@ export function renderModel(editor, model) {
  }
  });
  if (lines.length) {
  - removeNextSiblings(editor.children[lines.length]);
  + removeNextSiblings(editor.children[lines.length - 1]);
  } else {
  removeChildren(editor);
  }
  