Ignore missing screen for TurtlebatchCommand

Fixes CODEOCEAN-FRONTEND-47

app/assets/javascripts/editor/turtle.js

@@ -32,7 +32,7 @@ CodeOceanEditorTurtle = {
     this.showCanvas();
     for (var i = 0; i < msg.batch.length; i++) {
       var cmd = msg.batch[i];
-      this.turtlescreen[cmd[0]].apply(this.turtlescreen, cmd[1]);
+      this.turtlescreen[cmd[0]]?.apply(this.turtlescreen, cmd[1]);
     }
   },
 
@@ -50,4 +50,4 @@ CodeOceanEditorTurtle = {
     }
   }
 
-};
+};