Canvas绘图小程序问题：在模拟器上可以正常运行，但在PC端和手机端真机调试运行报错？
关于canvas绘图小程序问题：在微信开发者工具提供的模拟器上可以正常运行，但在PC端和手机端真机调试运行报错，哪位大牛帮忙看一下是什么原因，该如何解决 index.wxml关键点代码： <canvas id="canvas_preview" type="2d" mode="aspectFit" background-size="cover" style="vertical-align:middle; width:600px; height:600px"></canvas>
index.js关键代码：（参考小程序开发文档示例代码） wx.createSelectorQuery().select('#canvas_preview').fields({ id: true, node: true, size: true })
.exec((res) => {
var canvas = res[0].node
var ctx = canvas.getContext('2d')
......
})
真机运行报错信息如下： MiniProgramError Cannot read property 'getContext' of null TypeError: Cannot read property 'getContext' of null at Object.eval (weapp:///pages/index/index.js:239:38) at r.eval (eval at n.call.document (http://127.0.0.1:60808/remote-debug/runtime.js?devtools_ignore=true:1:14919), <anonymous>:2:1906677) at <SelectorQuery callback function> at eval (eval at n.call.document (http://127.0.0.1:60808/remote-debug/runtime.js?devtools_ignore=true:1:14919), <anonymous>:2:114523) at eval (eval at n.call.document (http://127.0.0.1:60808/remote-debug/runtime.js?devtools_ignore=true:1:14919), <anonymous>:2:1906803) at Array.forEach (<anonymous>) at eval (eval at n.call.document (http://127.0.0.1:60808/remote-debug/runtime.js?devtools_ignore=true:1:14919), <anonymous>:2:1906791) at eval (eval at n.call.document (http://127.0.0.1:60808/remote-debug/runtime.js?devtools_ignore=true:1:14919), <anonymous>:2:1906320) at eval (eval at n.call.document (http://127.0.0.1:60808/remote-debug/runtime.js?devtools_ignore=true:1:14919), <anonymous>:2:2673503) at A (eval at n.call.document (http://127.0.0.1:60808/remote-debug/runtime.js?devtools_ignore=true:1:14919), <anonymous>:2:2574511)
