I have a grid loaded well. I do a refresh of data and a refreah of the grid also going OK. Do a refresh again going OK. Do a change again and then I got an error on this.mydata  is null or not an object.
I checked this.getRowcount () ok. The data is in correspondence with all the parameters (at least I think so). There are no rows selected in the grid. There is no user interaction with the grid. The only thing what is happening is refresh the data array and try to display it.
After loading this.refresh () is called properly the error is in the toString method.