Spurious "Unknown address" messages from zyn first time file menu is accessed

[polluxsynth]

After starting zyn, and clicking FILE->LOAD INSTRUMENT in the GUI, zynaddsubfx emits the following:

Unknown addressBACKEND:online '/file_list_dirs:' Unknown addressBACKEND:online '/file_list_files:' Unknown addressBACKEND:online '/file_list_dirs:' Unknown addressBACKEND:online '/file_list_files:'

It doesn't seem damaging as the load operation still works.

[fundamental]

These messages are a known but fairly minor issue. Basically when listeners are setup on the UI side they realize that they don't have a value associated with the path they're observing, so they try to fetch it. In this case there is no zero argument version of those paths and the unknown address warning is generated. The issue needs to be fixed on the GUI side.