【报Bug】clearInterval偶尔会失效

代码如下：获取验证码之后 验证码按钮会进入倒计时 ，这个时候如果下拉刷新的话，清除验证码倒计时的方法clearInterval偶尔会失效 （getsendCode方法中的clearInterval没有失效过），请问如何解决呢？

        data() {  
            return {  
                timer: null,  
            };  
        },  
onPullDownRefresh() {  
            //清除验证码的倒计时  
            clearInterval(this.timer);  
            this.timer = null;  
        },  

//获取验证码  
            getsendCode() {  
                let that = this;  
                let countdown = 60;  
                that.$api('user.getInfo', {  
                    telephone: this.phone  
                }).then(res => {  
                    console.log(res)  
                    if (res.code === 200) {  
                        that.code.status = true;  
                        that.code.text = countdown + '秒后重新发送';  
                        that.code.status = true;  
                        this.timer = setInterval(() => {  
                            if (countdown > 0) {  
                                that.code.text = countdown - 1 + '秒后重新发送';  
                                countdown--;  
                            } else {  
                                clearInterval(this.timer);  
                                that.code.text = '获取验证码';  
                                that.code.status = false;  
                            }  
                        }, 1000);  
                    } else {  
                        that.code.status = false;  
                        console.log("错误信息=" + res.message);  
                    }  
                });  
            },