Fix exception after new payload file was saved

When a new payload file would be saved, the subsequent flight would always throw an exception (until Retribution would be restarted) because the payload_cache wasn't updated until now.

qt_ui/windows/mission/flight/payload/QLoadoutEditor.py

@@ -123,6 +123,7 @@ class QLoadoutEditor(QGroupBox):
                 f.write("local unitPayloads = ")
                 f.write(lua.dumps(payloads, indent=1))
                 f.write("\nreturn unitPayloads")
+            self.flight.unit_type.dcs_unit_type.add_to_payload_cache(payload_file)
         self.saved.emit(payload_name)
         QMessageBox.information(
             QWidget(),