IOS系统input设置maxlength时，输入到最后如果输入汉字的拼音长度超过限制会直接中断输入显示输入时的拼音

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)