想创建一个2d类型的canvas，采用微信官方文档中的范例，使用wx.createSelectorQuery().select(’#’).fields({ node: true, size: true }).exec(）语句，可调试总是出错，显示Cannot read property ‘node’ of null;at SelectorQuery callback function。TypeError: Cannot read property ‘node’ of null。

index.wxml文件

<view class="head-container">

<canvas type="2d" canvas-id="canvasIdPass" style="width: 300px; height: 300px;"></canvas>

</view>

index.js文件

onLoad:function(){

wx.createSelectorQuery().select('#canvasIdPass').fields({ node: true, size: true }).exec(function(res){

const canvas = res[0].node

const ctx = canvas.getContext('2d')

console.log('res[0]', res[0])

})

}