GridView does not assume that ModelData could be null.

[Mike Chaliy]

 
protected void detachWidgets(int startRow, int endRow, boolean remove) {
    if (endRow == -1) {
      endRow = ds.getCount() - 1;
    }
    for (int i = startRow; i <= endRow; i++) {
      detachWidget(grid.getStore().getAt(i)/* Here possible null*/, remove);
    }
  }
 
protected void detachWidget(ModelData modelData, boolean remove) {
   /* modelData could be null*/
    Map<Integer, Widget> m = modelData.get("gxt-widgetlist");
    if (m != null) {
      for (Widget w : m.values()) {
        ComponentHelper.doDetach(w);
      }
      if (remove) {
        modelData.set("gxt-widgetlist", null);
      }
    }
  }

I think better way is to prevent this in insert() method...