A nil argument for numChannels is presumably a result of a mistake. It can be hard to trace down.

E.g.:

b = Buffer(s); PlayBuf.ar(b.numChannels)

Returns [], because b has not yet been assigned a number of channels, so it is essentially PlayBuf.ar(nil).

I'd suggest that throwing an error would be better, even though I can imagine that in some cases, one could make use of the current behavior.

Fix:

in MultiOutUGen:

initOutputs { arg numChannels, rate;
        if(numChannels.isNil or: { numChannels < 1 }) { Error("%: wrong number of channels (%)".format(this, numChannels)).throw };
        channels = Array.fill(numChannels, { arg i;
            OutputProxy(rate, this, i);
        });
        if (numChannels == 1, {
            ^channels.at(0)
        });
        ^channels
    }