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關于繪圖小程序問題：在微信開發者工具提供的模擬器上可以正常運行微信小程序canvas圖片微信小程序canvas圖片，但在PC端和手機端真機調試運行報錯，哪位大牛幫忙看一下是什么原因，該如何解決

index.wxml關鍵點代碼：

<canvas?id="canvas_preview"?type="2d"?mode="aspectFit"?background-size="cover"?style="vertical-align:middle;?width:600px;?height:600px">canvas>

???index.js關鍵代碼：（參考小程序開發文檔示例代碼）

wx.createSelectorQuery().select('#canvas_preview').fields({?id:?true,?node:?true,?size:?true?})
????????????.exec((res)?=>?{
??????????????????????var canvas?=?res[0].node
??????????????????????var ctx?=?canvas.getContext('2d')

                                            ......
                              })

真機運行報錯信息如下：

read '' of null

: read '' of null

at .eval (weapp:///pages/index/index.js:239:38)

at r.eval (eval at n.call. (:60808/-debug/.js?=true:1:14919), :2:)

at

at eval (eval at n.call. (:60808/-debug/.js?=true:1:14919), :2:)

at eval (eval at n.call. (:60808/-debug/.js?=true:1:14919), :2:)

at Array. ()

at eval (eval at n.call. (:60808/-debug/.js?=true:1:14919), :2:)

at eval (eval at n.call. (:60808/-debug/.js?=true:1:14919), :2:)

at eval (eval at n.call. (:60808/-debug/.js?=true:1:14919), :2:)

at A (eval at n.call. (:60808/-debug/.js?=true:1:14919), :2:)