TypeError: Cannot read property 'push' of null

data(){
    return {
    ...
    form: {
        id: '',
        itemId: '',
        stem: '',
        optionList: null
      },
    ...
    }
}

methods: {
    handleAddOption() {
      var newLine = {
        option: ''
      }
      this.form.optionList.push(newLine)
    },
    ...
}

解決：push的時候，optionList沒有初始化，修改為optionList: []即可。