Fix segmentation fault in test cases

[Pietfried]

Describe your changes

With the introduction of the connectivity_manager in #748 the connectivity_manager was defined before the device_model in the ChargePoint class.

class ChargePoint : public ChargePointInterface, private ocpp::ChargingStationBase {
private:
    std::unique_ptr<ConnectivityManager> connectivity_manager;
    std::shared_ptr<DeviceModel> device_model;
    ...
}

This caused the destructor of the device_model to be executed before the destructor of the connectivity_manager.

DeviceModel::~
ConnectivityManager::~

This can lead to a segmentation fault in test cases where the ChargePointFixture was used. The websocket_disconnected_callback makes use of the device_model which can aleady be null at this time: https://github.com/EVerest/libocpp/blob/main/lib/ocpp/v201/charge_point.cpp#L3834

This PR changes the order of the definition of device_model and connectivity_manager so that the destructor of the ConnectivityManager is executed before the one of the DeviceModel.

Issue ticket number and link

#770

Checklist before requesting a review

• I have performed a self-review of my code
• I have made corresponding changes to the documentation
• If OCPP 2.0.1: I have updated the OCPP 2.0.1 status document
• I read the contribution documentation and made sure that my changes meet its requirements

[maaikez]

Shouldn't we also add a check in the websocket_disconnected_callback then maybe?

Other than that: well, it can be so small.... nice find!

[Pietfried]

Shouldn't we also add a check in the websocket_disconnected_callback then maybe?

Other than that: well, it can be so small.... nice find!

device_model should not happen to be null anymore, since the websocket destructor makes sure the (all) the callback(s) are executed before the object is destroyed