Playing multiple audio files at once in JUCE

I can’t seem to figure out how to play multiple files with no limit using JUCE. What I am having trouble with is when I run this getNextAudioBlock method below it returns an error saying: Built-in Output (9): EXC_BAD_ACCESS (code=1, address=0xc415f1d5b80)

CODE:

void getNextAudioBlock (const juce::AudioSourceChannelInfo& bufferToFill) override {
        if(rowNumSelected != 0){
            if (readerSource.get() == nullptr){
                bufferToFill.clearActiveBufferRegion();
                return;
            }
                             
            mixer.getNextAudioBlock (bufferToFill);
        }
    }

ENTIRE CODE:

https://github.com/ethanpvr18/Day-to-Dark