Fix crash when on a fixed rate tariff

If there are no charge windows then don't optimise any

apps/predbat/predbat.py

@@ -2297,7 +2297,7 @@ def update_pred(self):
         metric, self.charge_limit_percent, import_kwh_battery, import_kwh_house, export_kwh, soc_min, soc, soc_min_minute = self.run_prediction(self.charge_limit, self.charge_window, self.discharge_window, self.discharge_limits, load_minutes, pv_forecast_minute, save='base', end_record=end_record)
 
         # Try different battery SOCs to get the best result
-        if self.get_arg('calculate_best', False):
+        if self.get_arg('calculate_best', False) and self.charge_window_best:
             record_charge_windows = max(self.max_charge_windows(end_record + self.minutes_now, self.charge_window_best), 1)
             self.log("Record charge windows is {} end_record_abs was {}".format(record_charge_windows, self.time_abs_str(end_record + self.minutes_now)))