Hi...
I want to access to MailChimp data with OAuth 2.

I've found the following example at https://forceblogs.com/salesforce-integration-with-mailchimp-using-oauth-2-0/ but it doesn't work.

The error is because the line return null
var myPageRef = component.get("v.pageReference");

I appreciate any help

This is my modified code:
Helper

({
    doInit : function(component,Event,helper){
        var myPageRef = component.get("v.pageReference");

        console.log('here in doinit');

        console.log(myPageRef);
        var code = '';

        if(myPageRef != null){

           code = myPageRef.state.code;

           
        }

        console.log('this is the code'+code);
         
        var url = window.location.href;

        console.log(url);
        console.log(code);

        console.log(JSON.parse(myPageRef));
        if(code!=null && code.trim().length!=null && code!=undefined){
            var action=component.get("c.doFetchAccessToken");
            action.setParams({
                'code' : code,
                'redirectUri' : url.substr(0, url.indexOf('?'))
            });

            console.log('code '+code);
            console.log('redirectUri '+url.substr(0, url.indexOf('?')));
            action.setCallback(this, function(response){
                var state =response.getState();
                if(state==='SUCCESS'){
                    var res = response.getReturnValue();
                    component.set('v.accessToken',res);
                }
            });
            $A.enqueueAction(action);
        }
    },
    handleClick : function(component, event, helper) {
        var action=component.get("c.connectToMailchimp");
        action.setParams({
        });
        action.setCallback(this, function(response){
            var state =response.getState();
            if(state==='SUCCESS'){
                var res = response.getReturnValue();
                //console.log('URL FOR Mailchimp LOGIN-'+res.url);
                if(res.Error === undefined) {
                    console.log('Mailchimp Login Started');
                    window.open(res.url, '_top');
                }
                else if (res.Error != undefined && res.Error != null){
                    
                }
            } else if (state === 'ERROR'){
                console.log('Mailchimp Login Failed: ERROR');
            }
        });
        $A.enqueueAction(action);
    }
    
})

Client Controller

({	
    doInit : function(component, event, helper) {
		console.log('doinit ... ...');
    	helper.doInit(component, event, helper);
    },
	handleClick : function(component, event, helper) {
		helper.handleClick(component, event, helper);
	}
})

cmp

<aura:component implements="force:appHostable,flexipage:availableForAllPageTypes,flexipage:availableForRecordHome,force:hasRecordId,forceCommunity:availableForAllPageTypes,force:lightningQuickAction,lightning:isUrlAddressable" 
                access="global"
                controller = "MailChimpOAuth">
    <aura:attribute name="accessToken" type="String" />
    <aura:attribute name="pageReference" type="Object"/>
    <aura:handler name='init' value='{!this}' action='{!c.doInit}' />
    <div class = 'slds-grid slds-wrap slds-align_absolute-center' style="background-color:white;height:15rem;width:25rem;">
        <div class = 'slds-col slds-size_12-of-12 slds-align_absolute-center'>
            <lightning:button variant="brand" label="Connect To MailChimp" title="Base action" onclick="{! c.handleClick }"/><br></br>
        </div>
        <div class = 'slds-align_absolute-center'>
            Access Token is : {!v.accessToken}
        </div>
    </div>
</aura:component>

ApexClass
​​​​​​​

public class MailChimpOAuth {
    public static String clientId;
    public static String clientSecret;  
    @AuraEnabled
    public static Map<String, String> connectToMailchimp () {
        Map<String, String> returnMap = new Map<String, String>();

        MailChimp_Config__c gmc = getConfig();

        clientId = gmc.Client_ID__c;
        clientSecret = gmc.Secrect_ID__c;

        System.debug('logLevel');
        try{
            
            String redirectURI = 'https://speed-dream-189-dev-ed.lightning.force.com/lightning/n/Guije_Start';
            String url= 'https://login.mailchimp.com/oauth2/authorize';
            url+='?response_type=code&client_id='+clientId+'&redirect_uri='+ EncodingUtil.urlEncode(redirectURI, 'UTF-8');
            returnMap.put('url',url);
        } catch(Exception ex) {
            system.debug('Exception Error');
            system.debug(ex.getLineNumber()+':'+ex.getMessage()+':'+ex.getCause());
            returnMap.put('Error','Exception error while generating url'+'Line:'+ex.getLineNumber()+':Message '+ex.getMessage()+': Cause '+ex.getCause());
        }
        return returnMap;
    }   

    public static GUIJE_MailChimp_Config__c getConfig(){

        List<MailChimp_Config__c> gmc = [Select id, name, Client_ID__c, Secrect_ID__c, Code__c from MailChimp_Config__c];

        return gmc.get(0);
    }

    @AuraEnabled
    public static String doFetchAccessToken(String code, String redirectUri){

        MailChimp_Config__c gmc = getConfig();

        clientId = gmc.Client_ID__c;
        clientSecret = gmc.Secrect_ID__c;

        if(!String.isBlank(code)){

            gmc.code__c = code;

            update gmc;
        }
        

        System.debug('logLevel');
               
        String getTokenEndpoint = 'https://login.mailchimp.com/oauth2/token';
        String oAuthCode = code;
        String requestBody = 'grant_type=authorization_code&client_id='+clientId+'&client_secret='+clientSecret+'&redirect_uri='+EncodingUtil.urlEncode(redirectUri, 'UTF-8')+'&code='+code;
        String errorMessage ='';
        HttpRequest httpReq = new HttpRequest();
        HttpResponse httpRes = new HttpResponse();
        Http http = new Http();
        httpReq.setMethod('POST');
        httpReq.setEndPoint(getTokenEndpoint);
        httpReq.setBody(requestBody);
        
        try{
            httpRes = http.send(httpReq);
            
            if(httpRes.getStatusCode() == 200){
                Map<String, Object> response_Map = (Map<String, Object>)JSON.deserializeUntyped(httpRes.getBody());
                system.debug('response: '+httpRes.getBody());
                return String.valueOf(response_Map.get('access_token'));
            }else{
                system.debug('Unexpected Error with Mailchimp API'+
                             'Status '+httpRes.getStatus()+' and Status Code '+httpRes.getStatuscode());
            }
        }catch(System.Exception e){
            if(String.valueOf(e.getMessage()).startsWith('Unauthorized endpoint')){
                errorMessage = 'Unauthorize endpoint'
                    +'Goto Remote Site Setting and add '+' '+ getTokenEndpoint +' Endpoint';
                system.debug( errorMessage);
                //return null;
            }else{
                errorMessage = 'Unexpected Error'
                    +'Status '+httpRes.getStatus()+' and Status Code '+httpRes.getStatuscode();
                system.debug(errorMessage);
                //return null;
            }
        }
        return httpRes.getBody();
    }
}

Thanks,