Class to perform Callout
============================
public class BoxController  {
    
    
    public static  string clientid;
    public static  string  clientsecret;
    public static string redirecturi;
    public static string code;
    public  static   string jsonresponse;
    public  static string accessToken;
    public static string ViewAPI_Key;
    public  static string RefreshToken;
    public  static  string  jsonFolderResponse;
    public  static string  jsonFolderResponse1;
    public  static string jsonItemsResponse;
    public  static string folderId;
    public static String FileViewUrl;
    public static String securitycode;
    public static string box_login;
    public integer i=0;
    public BoxController()
    {
        //redirecturi='https://na30.salesforce.com/apex/boxresponse2';
        
        /* clientid=BoxCCredentials__c.getValues('CredentialsBox').Client_Id__c;  
clientsecret=BoxCCredentials__c.getValues('CredentialsBox').Client_Secret__c;
ViewAPI_Key=BoxCCredentials__c.getValues('CredentialsBox').View_API_Key__c;
AccessToken=BoxCCredentials__c.getValues('CredentialsBox').Access_Token__c;
RefreshToken=BoxCCredentials__c.getValues('CredentialsBox').Refresh_Token__c;
system.debug('Client_Id__c'+clientid);
system.debug('clientsecret'+clientsecret);
system.debug('clientsecret'+ViewAPI_Key);
system.debug('clientsecret'+AccessToken);*/
        
    }
    @future(callout=true)
    public static void  boxAuthentication(string aid)
    {
        try
        {
           
        String endpointUrl1='https://app.box.com/api/oauth2/authorize?response_type=code&client_id='+clientid+'&state=symcsk02&box_login=boxpocintegration@gmail.com';
        redirecturi='https://na30.salesforce.com/boxresponse2';
        code=ApexPages.currentpage().getparameters().get('code');// Attempt to dereference null object at  commented line in boxcontroller class
        securitycode=ApexPages.currentPage().getparameters().get('state');
        //  
        clientid=BoxCCredentials__c.getValues('CredentialsBox').Client_Id__c;  
        clientsecret=BoxCCredentials__c.getValues('CredentialsBox').Client_Secret__c;
        ViewAPI_Key=BoxCCredentials__c.getValues('CredentialsBox').View_API_Key__c;
        AccessToken=BoxCCredentials__c.getValues('CredentialsBox').Access_Token__c;
        RefreshToken=BoxCCredentials__c.getValues('CredentialsBox').Refresh_Token__c;
        system.debug('Client_Id__c'+clientid);
        system.debug('clientsecret'+clientsecret);
        system.debug('clientsecret'+ViewAPI_Key);
        system.debug('clientsecret'+AccessToken);
        
        HttpRequest request=new HttpRequest();
        request.setendpoint('https://api.box.com/oauth2/token'); 
        request.setmethod('POST');
        string body='grant_type=authorization_code&';
          body=body+'box_login'+box_login;  
        body=body+'code='+code;
        body=body+'&client_id='+clientid;
        body=body+'&client_secret='+clientsecret;
        request.setBody(body);
        Http p=new Http();
        HttpResponse response=p.send(request);
        jsonresponse=response.getBody();
        fromJSON js=(fromJSON)JSON.deserialize(jsonresponse,fromJSON.class);
        accessToken=js.access_token;
        system.debug('accessToken' +accessToken);
        
        system.debug('code is '+code);
        
        HttpRequest request1=new HttpRequest();
        request1.setendpoint('https://api.box.com/2.0/folders'); 
        request1.setmethod('POST');
        request1.setHeader('Authorization','Bearer ' + accessToken);
        string bbody='{"name":"a.name", "parent": {"id": "0"}} ';
        
        request.setBody(bbody);
        Http p1=new Http();
        HttpResponse response1=p1.send(request1);
        jsonFolderResponse=response1.getBody();
        system.debug('jsonFolderResponse'+jsonFolderResponse1);  
            }
        catch (exception ex)
        {
            system.debug('ex '+ex.getMessage()+'  '+ex.getLineNumber());
        }
    }
    
    public class fromJSON
    {
        public string access_token;
        public integer expires_in;
        public cls_restricted_to[] restricted_to;
        public string refresh_token;
    }
    class cls_restricted_to
    {
    }
}
==================
class which calls authorize page

public class WebserviceBox 
{
    public string clientid;
    public string code;
    
    public string AutorizeBox()
    {
        clientid=BoxCCredentials__c.getValues('CredentialsBox').Client_Id__c;
        String endpointUrl1='https://app.box.com/api/oauth2/authorize?response_type=code&client_id='+clientid+'&state=symcsk02';
        pageReference pag=new pageReference(endpointUrl1);
        pag.setRedirect(true);
        return endpointUrl1;
    }  
    public void restCode(){
         code=ApexPages.currentpage().getparameters().get('code');
    }
}
===========================
trigger for which calls the class

trigger BoxInsert on Account (after insert) 
{   
    for(account a:trigger.new)
    {
        try
        {
            
        WebserviceBox wc=new WebserviceBox();
        string code=wc.AutorizeBox(); 
        system.debug('code ' + code);
        BoxController.boxAuthentication(a.id);
            
        }
        catch (exception ex)
        {
            system.debug('ex '+ex.getMessage()+'  '+ex.getLineNumber());
        }
    }
 
}
================================
errror: Attempt to dereference null object at  commented line in boxcontroller class