Update app.py

app.py

@@ -640,9 +640,11 @@ async(c, o, b, p, d, n, m)=>{
       }
       } catch(e) {alert(e);}
     } else if (BABYLON || BABYLON == null) {
+    try {
       BABYLON = null;
-      document.getElementById("model3D").getElementsByTagName("canvas")[0].remove();
-      
+      if (document.getElementById("model3D").getElementsByTagName("canvas")[0]) {
+        document.getElementById("model3D").getElementsByTagName("canvas")[0].remove();
+      }
       document.getElementById("iframe3D").src = "index.htm";
       document.getElementById("iframe3D").onload = function() {
         document.getElementById("iframe3D").contentDocument.getElementById("coords").value = c;
@@ -671,6 +673,7 @@ async(c, o, b, p, d, n, m)=>{
       toggleDisplay("model");
       
       clearInterval(intv);
+    } catch(e) {alert(e)}
     }
   }, 40);
 }