Очевидно, adapterView.getSelectedItem()
возвращает GroupModel, а не FleetModel. Поэтому, если FleetModel не расширяет GroupModel, он выдаст ошибку приведения.
Какой тип данных в элементах вашего адаптера? Измените его на FleetModel или просто напишите GroupModel selectedItem = (GroupModel) adapterView.getSelectedItem();
Попробуйте это так:
spinnerFleet.setOnItemSelectedListener(new AdapterView.OnItemSelectedListener() {
@Override
public void onItemSelected(AdapterView<?> adapterView, View view, int i, long l) {
FleetModel selectedItem = stateAdapter.getItem(i); /*I think there was an error in this part of the code */
int vgID = displayIdData(selectedItem);
String jsonString = "{\"params1\":\"" + session_id + "\"," +
"\"params2\":\"5\"," +
"\"params3\":\"0\"," +
"\"params4\":\"0\"," +
"\"params5\":\"0\"," +
"\"params6\":\""+vgID+"\"," +
"\"params7\":\"0\"," +
"\"params8\":\"" + user_id + "\"}";
new LoadSpinnerGroupTask().execute(jsonString);
}