gantt.exportToExcel error

Gantt
#1

hello.
There is a problem when exporting to Excel.
(It worked fine before.)

If visual:true is removed, there is no problem, but if visual:true is used, Interner error occurs.

I will reproduce and share the situation in snippets.
http://snippet.dhtmlx.com/5/3e68ede67

P.S Error screen contents
=> Internal server error. Error: Gantt to Excel. TypeError: excelRefs variable sent to mergeCells function must be an array with length > 0

#2

Hello,
One of your tasks has a negative duration:

744×494 34 KB

http://snippet.dhtmlx.com/5/32465e496

Gantt allows adding tasks with these dates, but when you export the data to Excel with the visual: true parameter, the export server doesn’t expect that.
If you fix the duration and end_date parameters, the data will be exported correctly:

gantt.batchUpdate(function(){
  gantt.eachTask(function(task){
    if (task.duration < 0){
      task.duration = 0;
      task.end_date = task.start_date;
      gantt.updateTask(task.id)
    }
  })
})

Here is the updated snippet:
http://snippet.dhtmlx.com/5/3f125d44c

Could you share how you expect it to work with the negative duration when you try to export to Excel? Should the Gantt correct it to 0, remove the negative value, or throw an error?