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我正在使用 Microsoft Bot Framework 和 Vega 库构建数据可视化机器人。我遵循了 github 中的代码。谁能告诉我我的代码有什么问题？

但是，当我运行我的代码时，发生了错误：

/ - waterfall() step 1 of 1
/ - Session.send()
(node:11934) UnhandledPromiseRejectionWarning: Unhandled promise rejection (rejection id: 2): TypeError: Cannot read property 'getContext' of null
/ - Session.sendBatch() sending 1 message(s)

这是我的代码：

var sendVega = require('./botbuilder-vega');

// skip the setup server & create session process

function sendVegaChart(session){
    var chart = require('./test.vg.json');
    sendVega(session, "my chart", chart);
}

botbuilder-vega.js

var vega = require('vega');
var builder = require('botbuilder');

module.exports = function (session, messageText, vegaSpec) {
    var view = new vega
        .View(vega.parse(vegaSpec))
        .renderer('none')
        .initialize();

    view
        .toCanvas()
        .then(function (canvas) {
            var pngString = canvas
                .toBuffer()
                .toString('base64')
            var message = new builder
                .Message(session)
                .text(messageText);
            message.addAttachment({
                contentType: 'image/png',
                contentUrl: 'data:image/png;base64,' + pngString,
                name: 'Chart name'
            });
            // send message with attached image stream
            session.send(message);
        })
}
// END OF LINE

我的 json 文件看起来像这样——test.vg.json：

{
  "$schema": "https://vega.github.io/schema/vega/v3.0.json",
  "width": 500,
  "height": 200,
  "padding": 5,

  "data": [
    {
      "name": "table",
      "values": [
        {"x": 0, "y": 28, "c":0}, {"x": 0, "y": 55, "c":1},
        {"x": 1, "y": 43, "c":0}, {"x": 1, "y": 91, "c":1},
        {"x": 2, "y": 81, "c":0}, {"x": 2, "y": 53, "c":1},
        {"x": 3, "y": 19, "c":0}, {"x": 3, "y": 87, "c":1},
        {"x": 4, "y": 52, "c":0}, {"x": 4, "y": 48, "c":1},
        {"x": 5, "y": 24, "c":0}, {"x": 5, "y": 49, "c":1},
        {"x": 6, "y": 87, "c":0}, {"x": 6, "y": 66, "c":1},
        {"x": 7, "y": 17, "c":0}, {"x": 7, "y": 27, "c":1},
        {"x": 8, "y": 68, "c":0}, {"x": 8, "y": 16, "c":1},
        {"x": 9, "y": 49, "c":0}, {"x": 9, "y": 15, "c":1}
      ],
      "transform": [
        {
          "type": "stack",
          "groupby": ["x"],
          "sort": {"field": "c"},
          "field": "y"
        }
      ]
    }
  ],

  "scales": [
    {
      "name": "x",
      "type": "band",
      "range": "width",
      "domain": {"data": "table", "field": "x"}
    },
    {
      "name": "y",
      "type": "linear",
      "range": "height",
      "nice": true, "zero": true,
      "domain": {"data": "table", "field": "y1"}
    },
    {
      "name": "color",
      "type": "ordinal",
      "range": "category",
      "domain": {"data": "table", "field": "c"}
    }
  ],

  "axes": [
    {"orient": "bottom", "scale": "x", "zindex": 1},
    {"orient": "left", "scale": "y", "zindex": 1}
  ],

  "marks": [
    {
      "type": "rect",
      "from": {"data": "table"},
      "encode": {
        "enter": {
          "x": {"scale": "x", "field": "x"},
          "width": {"scale": "x", "band": 1, "offset": -1},
          "y": {"scale": "y", "field": "y0"},
          "y2": {"scale": "y", "field": "y1"},
          "fill": {"scale": "color", "field": "c"}
        },
        "update": {
          "fillOpacity": {"value": 1}
        },
        "hover": {
          "fillOpacity": {"value": 0.5}
        }
      }
    }
  ]
}
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