java - No properties to serialize found on class org.json.JSONObject when calling Firebase with getValue -

i have simple main activity gridview , every item has textview want fill 1 of contents of every item stored in firebase database. problem emerges when invoke getvalue(). app crashes , following error:

com.google.firebase.database.databaseexception: no properties serialize found on class org.json.jsonobject 

i using custom adapter made parse data gridview. code follows:

public class mainactivity extends appcompatactivity {       public jsonarray tables = new jsonarray();       @override      protected void oncreate(bundle savedinstancestate) {          super.oncreate(savedinstancestate);          setcontentview(r.layout.activity_main);           gridview grid = (gridview) findviewbyid(r.id.table_grid);           firebasedatabase firebasedatabase = firebasedatabase.getinstance();          databasereference databasereference = firebasedatabase.getreference("development/store_1/tables");          databasereference.child("items").addvalueeventlistener(new valueeventlistener() {              @override              public void ondatachange(datasnapshot datasnapshot) {                  iterable<datasnapshot> children = datasnapshot.getchildren();                   (datasnapshot child : children) {                      jsonobject table = child.getvalue(jsonobject.class);                 tables.put(table);             }         }               @override              public void oncancelled(databaseerror databaseerror) {                   string error = databaseerror.getmessage().tostring();                   toast.maketext(mainactivity.this, error, toast.length_short).show();              }          });           tableadapter tableadapter = new tableadapter(this, tables, 0);          grid.setadapter(tableadapter);      }  }