Cannot read property 'classList' of undefined

[kangzhongyao]

MessageBox.prompt('',mess,'').then(action => {
this.myobj.taskId = a
this.myobj.employeeId = b
this.myobj.status = c
http.get('/api/pcEmTaskAssignmentController/updateStatusPc',this.myobj,).then((data)=> {

            })
        });