Try changing the "type" field of your audio source from "Float" to "Complex".
I am working on an update of op25 to use the new tools present in the latest GNURadio (cmake, grc support, new namespace rules, message passing, stream meta-information and so on) . I am particularly impressed by GRC as a means for building radios and want to see this side of our project improved because I think it allows non-programmers to build radios and play around more easily.