distanceTool.undo() error

[whiteMelody]

maptalks v0.42.0

distanceTool.undo()
vue.esm.js?a026:1897 TypeError: Cannot read property 'slice' of undefined
at t.undo (maptalks.min.js:6)
at VueComponent.repeal (Tools.vue?e44b:1054)
at invokeWithErrorHandling (vue.esm.js?a026:1863)
at HTMLDivElement.invoker (vue.esm.js?a026:2188)
at HTMLDivElement.original._wrapper (vue.esm.js?a026:7565)

内部项目，暂时无法提供地址

[fuzhenn]

reproduction:
https://codepen.io/whitemelody/pen/qBOGmvz

[fuzhenn]

结束绘制后点undo会报错，调用undo时需要判断是否已经结束绘制即可