uni-app动态修改导航栏的buttons 的text？

刚进入点击编辑可改成完成，再点完成就报错了，
https://ask.dcloud.net.cn/question/71175 这个链接我看的不是很懂，试了没弄出来，希望有具体的案例，非常感谢

    onNavigationBarButtonTap(e) {  
            console.log(e)  
            const index = e.index;  
            console.log(this)  
            console.log(this.$mp.page)  
            if (index === 0) {  
                if (e.text === "编辑") {  
                    var currentWebview = this.$mp.page.$getAppWebview();  
                    currentWebview.setTitleNViewButtonStyle(0, {  
                        text: "完成",  
                    });  
                    // this.titleNViewWebview()  
                    this.showChecked = true;  
                } else {  
                    var currentWebview = this.$mp.page.$getAppWebview();  
                    currentWebview.setTitleNViewButtonStyle(0, {  
                        text: "编辑",  
                    });  
                    // this.titleNViewWebview()  
                    this.showChecked = false;  

                }  

            }  
        },  

报错  

[Vue warn]: Property or method "toJSON" is not defined on the instance but referenced during render. Make sure that this property is reactive, either in the data option, or for class-based components, by initializing the property. See: https://vuejs.org/v2/guide/reactivity.html#Declaring-Reactive-Properties.  
15:10:46.897 (found in pages/my/family)