Addendum: the filter error is something I saw once before. For some unknown reason your
version of gnuradio does not have the fft_filter_ccf() routine, so you are forced to use
fir_filter_ccf() instead. Max's code uses the FIR version by default, but I'd
changed mine to use FFT because it was much more cpu-efficient for the lightweight
hardware such as Rasp PI 3.
You can edit the p25_demodulator.py file, line 173 and change 'fft' to
'fir' and it should work on your system.