As far as I could find out for now, variables may only be accessed in a generic way by calling "getAvailableFlowVariables()" on the NodeModel superclass which delivers a map of all variables connected to the node.
In my use case, the input port of type FlowVariableObjectPort is meant to connect a variable for a special purpose. I just want the user to connect a variable (String) to the port, which later serves as a path. I do not know the name of the variable, neither do I want the user to follow a specified name (only the value matters). The FlowVariableObjectPort itself seems to be just a connection markup.
So how can I access the very variable from the input port (not knowing the name)?