Why do I get an error when I try to play two Audio Files at the same time?

[ethanpvr18]

I am trying so hard to play two or more audio files at the same time using Juce. So far I have made my own class called TransportMixerAudioSource, and in it I extended MixerAudioSource for Transport Sources to be accepted. However, I am getting an error in the getNextAudioBlock Method. The Code is as follows:

ERROR:

//========================================
    //Section [2] -- Playing Audio Helper Methods
    void prepareToPlay (int samplesPerBlockExpected, double sampleRate) override {
         mixer.prepareToPlay (samplesPerBlockExpected, sampleRate);
    }
    void releaseResources() override {
        mixer.releaseResources();
    }
    void getNextAudioBlock (const juce::AudioSourceChannelInfo& bufferToFill) override {
        if(rowNumSelected != 0){
            if (readerSource.get() == nullptr){
                bufferToFill.clearActiveBufferRegion();
                return;
            }
            mixer.getNextAudioBlock(bufferToFill);
        }
    }
    //========================================  
bool keyPressed(const KeyPress &k, Component *c) override {
        //Subsection [g] -- Method Part (a) to Play Cue when selected and pressed
        if( k.getKeyCode() == juce::KeyPress::spaceKey ) {
            auto file = getAttributeFileForRowId(rowNumSelected-1);
            auto* reader = formatManager.createReaderFor (file);
            if (reader != nullptr) {
                std::unique_ptr<juce::AudioFormatReaderSource> newSource (new juce::AudioFormatReaderSource (reader, true));
                AudioTransportSource* newCue = new AudioTransportSource();
                newCue->setSource(newSource.get(), 0, nullptr, reader->sampleRate);
                mixer.addInputSource(newCue, false);
                readerSource.reset (newSource.release());
            }
            if(isPlaying)
                mixer.stop();
            mixer.start();
            file = "";
            reader = nullptr;
            table.selectRow(rowNumSelected, false, true);
            isPlaying = true;
            return true;
        }
        //Subsection [h] -- Method Part (b) to Stop Cue when selected and pressed
        if( k.getKeyCode() == juce::KeyPress::escapeKey ) {
            mixer.stop();
            isPlaying = false;
            return true;
        }
            
        return false;
    }
//===============================================

This is the TransportMixerAudioSource