Accessing single element in shared pointer

Hi all! I'm trying to access the final circle in this ofxBox2d joint and have it rotate around my mouse; ultimately I want it to look as if it was coiling around it but I'm not sure how to get there. As I mentioned, I'm working with ofxBox2d to try to accomplish this.

I was thinking I could access it the same way I would access a specific element in any array, by just saying (in this case)

circles[30].get()->setPosition(x, y);

but I am getting a getting a green highlight that says

EXC_BAD_ACCESS (code=EXC-l386_GPFLT)

I tried to debug, and it was recommended I enable Zombie objects, but I don't think the output has changed. Why is this an incorrect method of attempting to do this?

Here is the code in the cpp file:

#include "ofApp.h"

//--------------------------------------------------------------
void ofApp::setup(){
    ofSetVerticalSync(true);
    ofBackgroundHex(000000);
    ofSetLogLevel(OF_LOG_NOTICE);
    ofDisableAntiAliasing();
    
    box2d.init();
    box2d.setGravity(0, 0);
    box2d.createBounds();
    box2d.setFPS(60.0);
    box2d.registerGrabbing();
    
//  anchor.setup(box2d.getWorld(), ofGetWidth()/2, ofGetHeight()/2, 4);
    
    //add a few circles
    for (int i = 0; i < 30; i++)
    {
        shared_ptr<ofxBox2dCircle> circle = shared_ptr<ofxBox2dCircle>(new ofxBox2dCircle);
        circle.get()->setPhysics(3.0, 0.53, 0.1);
        circle.get()->setup(box2d.getWorld(), ofGetWidth()/2, 100+(i*20), 8);
        circles.push_back(circle);
    }
    
    //connect each circle with joint
    for (int i = 0; i < circles.size(); i++)
    {
        shared_ptr<ofxBox2dJoint> joint = shared_ptr<ofxBox2dJoint>(new ofxBox2dJoint);
        
        //if first point, connect to top anchor
        if (i == 0)
        {
            joint.get()->setup(box2d.getWorld(), anchor.body, circles[i].get()->body);
        }
        else
        {
            joint.get()->setup(box2d.getWorld(), circles[i-1].get()->body, circles[i].get()->body);
        }
        
        joint.get()->setLength(25);
        joints.push_back(joint);
    }
}

//--------------------------------------------------------------
void ofApp::update(){
    box2d.update();
    
    timeElapsed = ofGetElapsedTimef();
    
    ofPoint mouse(ofGetMouseX(), ofGetMouseY());
    
    //"friendly" values: 25, 50
    float repelDist = 150;
    float attractDist = 400;
    
    for (int i = 0; i < circles.size(); i++)
    {
        float dist = mouse.distance(circles[i].get()->getPosition());
        float force = ofMap(dist, 0, 1262, 1.3, 0.1);
//        circles[i].get()->addRepulsionForce(mouse, force);
            if (dist < repelDist)
            {
//              circles[i].get()->addRepulsionForce(mouse, 5.5);
            }
            if (dist > attractDist)
            {
//                circles[i].get()->addAttractionPoint(mouse, 1.2);
            }
        //std::cout << dist << endl;
    }
    
}

//--------------------------------------------------------------
void ofApp::draw(){
    
    ofSetHexColor(0xf2ab01);
    ofSetLineWidth(5);
    
//    anchor.draw();
            for (int i = 0; i < circles.size(); i++)
            {
                ofFill();
                ofSetHexColor(0x01b1f2);
                
                float radius = 1.0;
                for (int deg = 0; deg < 360*6; deg += 10)
                {
                    angle = ofDegToRad(deg);
                    float x = ofGetMouseX() + sin(angle) * radius;
                    float y = ofGetMouseY() + cos(angle) * radius;
                    circles[30].get()->setPosition(x, y);
                }
                
                circles[i].get()->draw();
                
                std::cout << circles[i].get()->getPosition() << endl;
                
            }
    
            for (int i = 0; i < joints.size(); i++)
            {
                ofSetHexColor(0xFFFFFF);
                joints[i].get()->draw();
            }
    
    string info = "";
    
}