Check the port handler is non nullptr before trying to close it

[Jack-Punter]

Fixes #404 by first validating the port_handler_ is not null.
I haven't looked at the sdk code so i'm going to assume that we don't need to free this pointer, but its unclear whether this is needed or not from the headers.

[gemini-code-assist]

Summary of Changes

Hello @Jack-Punter, I'm Gemini Code Assist¹! I'm currently reviewing this pull request and will post my feedback shortly. In the meantime, here's a summary to help you and other reviewers quickly get up to speed!

This pull request enhances the robustness of the DynamixelDriver by addressing a potential null pointer dereference. It ensures that the portHandler_ is valid before attempting to close the port during object destruction, thereby preventing runtime errors and improving the stability of the driver.

Highlights

• Null Pointer Dereference Prevention: A null pointer check has been added for portHandler_ in the DynamixelDriver destructor. This prevents a potential crash if portHandler_ was not initialized before closePort() is called.

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[gemini-code-assist]

Code Review

This pull request correctly adds a null check for portHandler_ in the DynamixelDriver destructor before calling closePort(), which prevents a potential crash. This is a good defensive programming practice.

Regarding your question about memory management: you are correct not to delete portHandler_. The dynamixel::PortHandler::getPortHandler() method returns a pointer to a singleton instance managed by the SDK, so you don't need to manually free it.

However, for a complete and robust fix, two related issues should be addressed:

1. Uninitialized Pointer: The member pointer portHandler_ is not initialized in the DynamixelDriver constructor. In C++, uninitialized member pointers hold garbage values, which can cause the portHandler_ != nullptr check to behave unpredictably. Please initialize portHandler_ to nullptr in the constructor's initializer list to ensure this check is always safe.

2. Missing Null Check in setPortHandler: In the setPortHandler method, the return value of dynamixel::PortHandler::getPortHandler(device_name) is not checked for nullptr before being used. This could lead to a crash if getPortHandler fails. A null check should be added there as well.

While this PR is a good step, addressing these additional points will make the code more robust.