what could be cause of this error

When doing Drag n Drop from one grid on page to a tree grid on same page, I see following error on firefox error console sometimes.
It does not impact the functionality and User does not see any error. Just wondering what could be possible cause?

Error: c is null
Source File: /media/dhtmlx/dhtmlxGrid/codebase/dhtmlxgrid.js
Line: 856

var c = this.getRowById(row_id);var cell = (c._childIndexes ? c.childNodes[c._childIndexes[col]] : c.childNodes[col]);return this.cells4(cell);},

If issue still occurs and you can provide a sample - please send it to the support@dhtmlx.com
Locally it works correctly, so it is unclear what is the reason of the problem.