Error Occurred: An Apex error occurred: System.NullPointerException: Attempt to de-reference a null object

public class sendAnEmail
{
     
      @InvocableMethod( label='Test' description='sends an email')
       
    public static void sendEmailWithAttachment(List<id> listofQuoteHeader)
    {   
      
        // Application__c applink = [SELECT Id,Name FROM Application__c  ];
        Map<Id, Application__c> quotesMap =  new Map<Id, Application__c>([SELECT id,Contact__r.Email,Contact__r.Name FROM Application__c WHERE Id IN :listofQuoteHeader]);
           for(Id QuoteHeaderid :listofQuoteHeader)
           {
               PageReference pref= page.PDFGEN;
               pref.getParameters().put('id',(Id)QuoteHeaderid);
               pref.setRedirect(true);
                system.debug('--appID-'+QuoteHeaderid);
               Attachment attachment = new Attachment();      
               Blob b=pref.getContentAsPDF();
               attachment.Body = b;
               attachment.Name = Datetime.now().format('yyyy-MM-dd HH:mm') + ' ' + 'Quote' + '.pdf';
               attachment.IsPrivate = false;
               attachment.ParentId = QuoteHeaderid;
               attachment.Name='Sign.png';
               insert attachment;
               
               Messaging.SingleEmailMessage semail= new Messaging.SingleEmailMessage();
               Messaging.EmailFileAttachment attach= new Messaging.EmailFileAttachment();
               attach.setFileName('AttachmentEmailFile.pdf');
               attach.setBody(b);
               semail.setSubject('Quote Issued');
            //  String[] emailIds= new String[]{'abc@gmail.com'}; 
                String[] emailIds= new String[]{quotesMap.get(QuoteHeaderid)?.Contact__r.Email};
               semail.setToAddresses(emailIds);
               semail.setPlainTextBody('Please find the attached quote details');
 semail.setHtmlBody('Hello ' +  + '<br/><br/>Summary report' + contact.Name+'.<br/><br/><a href="'+ URL.getSalesforceBaseUrl().toExternalForm() + '/' + QuoteHeaderid+'">Click Here</a> Please check your Summary report');

               semail.setFileAttachments(new Messaging.EmailFileAttachment[]{attach});
                Messaging.sendEmail(new Messaging.SingleEmailMessage[]{semail});
                  system.debug('----'+listofQuoteHeader);
           } 
        
 SendAnEmail.sendAnEmail();// I have call the constructor method in invocable method
    }
    // this method for merge fields
       private static  Application__c app{get; private set;}
       public static void sendAnEmail() {
          
               Id id = ApexPages.currentPage().getParameters().get('id');
                app = (id == null) ? new Application__c() : 
            [SELECT id,Name FROM Application__c WHERE Id = :id];
           }
           
}

<apex:page  controller="sendAnEmail" renderAs="advanced_pdf" applyHtmlTag="false" applyBodyTag="false" standardStylesheets="false"
showHeader="false" sidebar="false">>
   <apex:form >
             You belong to app Name:{!app.Name}
          </apex:form> 
    
     <h1>
    
      hello
    </h1>
  
</apex:page>