Insert a picture at Range(0,0) and drag it left or top will cause null pointer exception

1. Use the attach file "test.zul"
2. click insert picture button
3. drag picture left or top will cause NPE

It cause by client sending null value to server
data_1:

{"sheetId":"z3_sheet_8","wgtType":"image","action":"move","wgtId":"z3_pic_0","dx1":null,"dy1":26,"dx2":null,"dy2":13,"col1":null,"row1":0,"col2":null,"row2":4}