GWT 2.8 dropevent datatransfer null

Question

I'm trying to drag and drop an element from a DataGrid which is the widget of a FocusPanel. The target is another DataGrid which is the widget of another FocusPanel contained in a DialogBox. The method onDragStart works fine. I can check with the debugger (Firefox 53.0) that the dataTransfer object is populated with the setData method of the DragStartEvent. On the side of the target the onDrop method is correctly called. The issue is that the dataTransfer object is null and the getData method of DropEvent returns an error. I don't understand why the dataTransfer object is null.

Answer 1

I found the answer when reading the paragraph "Performing a Drop" on MDN "Drag Operations". There is a js code snippet where event.preventDefault() is placed at the end of the onDrop function. This drew my attention since my Java code is calling preventDefault() in the beginning of the onDrop() method. I moved the call to preventDefault() to the end of the method and it works. The dataTransfer is no more null and getData() returns the correct information.