There seems to be some problem with naming the method:

fun float last() (line 48)

When I try to plug the value from that method into a ugen, I get an instance VM crash.

When renaming to lastT() for example, the EnvelopeFollower class functions as normal. Perhaps there is a conflict with the inherited Chugraph class' last() method?

adc => EnvelopeFollower env => blackhole;
SinOsc s => dac;

while (true)
{
    100::ms => now;
   env.last() => s.gain;
    <<<env.last()>>>;
}