Update app.py

app.py

@@ -797,10 +797,12 @@ function drawLine(x, y) {
 window.drawLine = drawLine;
 
 
-document.getElementById("image_edit").getElementsByTagName("canvas")[0].oncontextmenu = function(e){e.preventDefault();}
-document.getElementById("image_edit").getElementsByTagName("canvas")[0].ondrag = function(e){e.preventDefault();}
+var intv_ = setInterval(function(){
+if (document.getElementById("image_edit") && document.getElementById("image_edit").getElementsByTagName("canvas")) {
+  document.getElementById("image_edit").getElementsByTagName("canvas")[0].oncontextmenu = function(e){e.preventDefault();}
+  document.getElementById("image_edit").getElementsByTagName("canvas")[0].ondrag = function(e){e.preventDefault();}
             
-document.getElementById("image_edit").getElementsByTagName("canvas")[0].ondblclick = function(e) {
+  document.getElementById("image_edit").getElementsByTagName("canvas")[0].onclick = function(e) {
     var x = parseInt((e.clientX-e.target.getBoundingClientRect().x)*e.target.width/e.target.getBoundingClientRect().width);
     var y = parseInt((e.clientY-e.target.getBoundingClientRect().y)*e.target.height/e.target.getBoundingClientRect().height);
 
@@ -813,7 +815,10 @@ document.getElementById("image_edit").getElementsByTagName("canvas")[0].ondblcli
     evt.initEvent("input", true, false);
     document.getElementById("mouse").getElementsByTagName("textarea")[0].dispatchEvent(evt);
     document.getElementById("mouse").getElementsByTagName("textarea")[0].value = "[]";
+  }
+  clearInterval(intv_);
 }
+}, 40);
 
 }
 """