在promise axios.all中找不到数据

Problem is that (this.form.tasks neither other data) cannot be found in the promise then() with axios.all. Outside the promise it works but not inside. I get the error :

Uncaught (in promise) TypeError: Cannot read property 'form' of null

        axios.all([
            axios.get('/tasks/'+this.token_proj),
            axios.get('/keywords/'+this.token_proj),
            axios.get('/getTasks/'+this.token_proj),
            axios.get(`/discussion/${this.token_proj}/${this.form.page}`)
        ])
        .then(axios.spread(function (tasksResponse, keywordsResponse,getTasksResponse,discResponse) {

            console.log('tasks', tasksResponse.data);
            console.log('keyword', keywordsResponse.data);
            console.log('getrask', getTasksResponse.data);
            console.log('disc', discResponse.data);

            this.form.tasks = tasksResponse.data;



        }));

The tasks variable is set in data and exist. I use Vuejs and axios vform.

data(){
        return{
            form: new Form({

            tasks:[],
            tasks_management:[],
            discuss:{},
            users:[],
            keywords:[],


            })
        }
    }