React Uncaught Invariant Violation: onlyChild must be passed a children with exa

报错提示：

Uncaught Invariant Violation: onlyChild must be passed a children with exactly one child.

报错代码：

pmessage = this.state.showmessage?<div className="pmessage">{this.state.pmessage}</div>:"";

return (
      <div className="userinfo">
        <TimeoutTransitionGroup enterTimeout={450} leaveTimeout={450} transitionName="pmessage" className='pmessage' >
          { pmessage }
        </TimeoutTransitionGroup>
      </div>
    );

解决方案：

pmessage = this.state.showmessage?<div className="pmessage">{this.state.pmessage}</div>:<div></div>; // 添加child DOM


return (
      <div className="userinfo">
        <TimeoutTransitionGroup enterTimeout={450} leaveTimeout={450} transitionName="pmessage" className='pmessage' >
          { pmessage }
        </TimeoutTransitionGroup>
      </div>
    );

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