OK @stephenk here is my first workaround
class Arpeggiator : public AudioProcessor
{
public:
//==============================================================================
Arpeggiator()
: AudioProcessor (BusesProperties().withInput("Input", AudioChannelSet::mono(), true)) // seem to need for sampleRate
// : AudioProcessor(BusesProperties()) // add no audio buses at all - Original, doesn't work
{
addParameter (speed = new AudioParameterFloat ("speed", "Arpeggiator Speed", 0.0, 1.0, 0.5));