Assertion error in AudioProcessor when it only has output but not input

I’m trying to use a simple sampler:

: AudioProcessor (BusesProperties()
.withOutput (“Output”, juce::AudioChannelSet::stereo(), true)
), mAPVTS (*this, nullptr, “Parameters”, createParameters())
mAPVTS.state.addListener (this);

for (int i = 0; i < mNumVoices; i++)
    mSampler.addVoice (new juce::SamplerVoice());


but I get

I/JUCE: JUCE Assertion failure in juce_AudioProcessor.cpp:200
I/JUCE: JUCE Assertion failure in juce_AudioProcessor.cpp:350
I/JUCE: JUCE Assertion failure in juce_AudioProcessor.cpp:356
JUCE Assertion failure in juce_AudioProcessor.cpp:360
I/JUCE: JUCE Assertion failure in juce_AudioProcessor.cpp:363

if I add

                 .withInput ("Input", juce::AudioChannelSet::stereo(), true)

then I don’t get the assertion errors anymore, but then I get an echo because of the input. I don’t want my audio processor to have an input, I just want to process midi.

I’m on Android. The assertion error is immediately after creating an instance of this audio processor