The below code runs fine in testing environment, it's actually generating a PDF document.

public with sharing class PDFTransactionSlip {
    public PDFTransactionSlip() {
    }
   
    public PDFTransactionSlip(ApexPages.StandardController controller) {
    }

    private String getXmlString(pb__Agreement__c c)
    {
       //datetime todayis = system.now();
       
       double skcomdue = c.Buyer_Agency_Fee__c + c.Seller_Agency_Fee__c; 
       double premiumseller = c.pb__PurchaseListPrice__c - c.Inventory_Original_Price__c;
       String s = '<?xml version="1.0" encoding="UTF-8"?>' +
           '<xfdf xmlns="http://ns.adobe.com/xfdf/" xml:space="preserve">' +
           '<f href="https://na*.salesforce.com/resource/1298882850000/Transaction_Slip"/>' +
           '<fields>' +
           '<field name="Transaction_Type"><value>' + c.pb__AgreementType__c + '</value></field>' + 
           '<field name="Buyer_Type"><value>' + c.Buyer_Type__c + '</value></field>' + 
	        '<field name="Buyer_Name"><value>' +c.Buyer_Name__c + '</value></field>' +
	        '<field name="Buyer_No"><value>' + c.Buyer_Phone_No__c + '</value></field>' +
	        '<field name="Buyer_Address"><value>' + c.Buyer_Address__c + '</value></field>' +
	  		'<field name="Buyer_Pasport"><value>' + c.Buyer_Passport_No__c+ '</value></field>' +
	        '<field name="Buyer_Nationality"><value>' + c.pb__AccountRegistrationCountry__c + '</value></field>' + 
	         '<field name="Seller_Type"><value>' + c.Seller_Type__c + '</value></field>' + 
	        '<field name="Seller_Name"><value>' + c.pb__SellerFullName__c + '</value></field>' +
	        '<field name="Seller_No"><value>' + c.seller_Phone_no__c + '</value></field>' +
	        '<field name="Seller_Address"><value>' + c.Seller_Address__c + '</value></field>' +
	  		'<field name="Seller_Pasport"><value>' + c.Seller_Passport_No__c + '</value></field>' +
	        '<field name="Seller_Nationality"><value>' + c.Seller_Country__c + '</value></field>' + 
	         '<field name="Developer"><value>' + c.pb__DeveloperRegistrationNumber__c + '</value></field>' + 
	        '<field name="PSA Date"><value>' + c.pb__AgreementDate__c + '</value></field>' +
	        '<field name="Unit_Details"><value>' + c.pb__UnitName__c + '</value></field>' +
	  		'<field name="No_Of_Bedrooms"><value>' + c.pb__UnitBedrooms__c + '</value></field>' +
	        '<field name="Sqft"><value>' + c.pb__TotalAreaSqft__c + '</value></field>' +   
	        '<field name="Built_Up"><value>' + c.pb__TotalAreaSqft__c + '</value></field>' + 
	         '<field name="Original_Price"><value>' + c.Inventory_Original_Price__c + '</value></field>' + 
	        '<field name="Net_Purchase_Price"><value>' + c.pb__PurchaseListPrice__c + '</value></field>' +
	        '<field name="Amount_Paid"><value>' + c.Amount_Paid_on_Property__c + '</value></field>' +
	        '<field name="Premium_Seller"><value>' + premiumseller + '</value></field>' +
	  		'<field name="Deposit Amount"><value>' + c.Deposit_Amount__c + '</value></field>' +
	        '<field name="Transfer_Fee"><value>' + c.Transfer_Fee__c + '</value></field>' + 
	         '<field name="Total_Purchase"><value>' + c.Total_Purchase_Price_AED__c + '</value></field>' + 
	        '<field name="Buyer_Agency"><value>' + c.Buyer_Agency_Fee__c + '</value></field>' +
	        '<field name="Seller_Agency"><value>' + c.Seller_Agency_Fee__c + '</value></field>' + 
	        '<field name="Conveyance_Fees"><value>' + c.Conveyance_Fees__c  + '</value></field>' +
	  		'<field name="SK_Commission_Due"><value>' + skcomdue  + '</value></field>' +
	  		'<field name="Total_Commission"><value>' + skcomdue+ '</value></field>' +             
           '</fields><ids original="6CF3C495DE77414F85BC24E6CFC3F692" modified="EF5CB2FB7A5BAA4FB634933ADC944031"/>' +
           '</xfdf>';
                    
       return s;
    }
    
    public PageReference XFDFInit() 
    {
        LIST<pb__Agreement__c> c = [Select Agreed_Price__c,pb__AgreementType__c,
        pb__AccountRegistrationCountry__c , Amount_Paid_on_Property__c, Buyer_Agency_Fee__c, 
    Buyer_Conveyance_Fee__c,Buyer_Phone_No__c, Buyer_Address__c, Buyer_Passport_No__c,
    Buyer_Type__c, Cheque_Data__c, Conveyance_Fees__c, 
    Deposit_Amount__c,Buyer_Name__c, Deposit_Status__c, Id, Initial_Term_SKadded__c, 
    Inventor_Title_No__c, Inventory_Completion_Status__c, Inventory_Original_Price__c,
    Landlord_Commision__c, Mortgage_Discharge_Fee__c, Mortgage_Registration_Fee__c,
    Name, Net_Achievable_Price_to_Seller1__c, NOC_Fee_to_Developer__c,
    Number_of_Cheques_Required__c, pb__AccountId__c, pb__DeveloperId__c,
    pb__DeveloperRegistrationNumber__c, pb__DownPaymentsComplete__c,
    pb__FeesComplete__c, pb__InventoryId__c, pb__ItemCompletionStatus__c,
    pb__OfferId__c,seller_phone_no__c, pb__PurchaseListPrice__c, pb__PurchasePrice__c,
    pb__SellerFullName__c, pb__SellerId__c, pb__Status__c, pb__TotalAmount__c,
    pb__TotalAreaSqft__c, pb__TotalAreaSqm__c, pb__UnitBedrooms__c, 
    pb__UnitFloorNumber__c, pb__UnitName__c, pb__UnitType__c, 
    Seller_Address__c, Seller_Agency_Fee__c, Seller_Conveyance_Fee__c, 
    Seller_Country__c, pb__AgreementDate__c , Seller_Passport_No__c, Seller_Type__c,
    Service_Maintenance_Fee_Pro_rata__c, Tenant_Commission__c, 
    Title_Deed__c, Total_Purchase_Price_AED__c, Transfer_Date__c, 
    Transfer_Fee__c, Unit_Number__c from pb__Agreement__c 
        WHERE id = :ApexPages.currentPage().getParameters().get('id') limit 1];
       
        
        
        
        
      if(c.size() > 0){
      	pb__Agreement__c nc = c[0];
        String xmlContent = getXmlString(nc);      
        Attachment attachment = new Attachment();
        attachment.Body = Blob.valueOf(xmlContent);
        attachment.Name = 'Transaction_Slip_' + c[0].Name + '_' + System.now() + '.XFDF';
        attachment.ParentId = c[0].Id;
        insert attachment;                 
        PageReference contactPage = new PageReference('/' + c[0].id);
        contactPage.setRedirect(true);
        return contactPage;
      }
	  PageReference contactPage = new PageReference('/' + ApexPages.currentPage().getParameters().get('id'));
      contactPage.setRedirect(true);
      return contactPage;
   }  
}

Problem is when try to execute it in production, it gives the following exception:

System.NullPointerException: Attempt to de-reference a null object

Class.PDFTransactionSlip.getXmlString: line 12, column 26 Class.PDFTransactionSlip.XFDFInit: line 85, column 29 External entry point