DroneCAN Params window fails to "Save to file" for string parameters

DroneCAN (aka UAVCAN) supports string parameters. The Xacti gimbal is one device that makes use of this ability.

The UAVCAN Params window's "Save to file" fails with error message when it tries to save string parameters.

Start in the UAVCAN Params window at the "Save to file" action and reproduce the failure with a string parameter, such as one from the Xacti gimbal. Trace the save path to the error shown in the issue; done means string parameters save successfully without breaking other parameter types.

Written by the indexing model from the issue text.