Hi
I have created a apex class for uploading file.but i am uplaoding any files its showing this error.how to achieve this error ?


System.NullPointerException: Attempt to de-reference a null object
Error is in expression '{!doUploadFile}' in component <apex:commandButton> in page scoupload: Class.SCOUpload.doUploadFile: line 74, column 1
Class.SCOUpload.doUploadFile: line 74, column 1
 

public with sharing class SCOUpload {
 
    /*
    *   attachment
    */
    public Purchase_Order__c purchaseOrder{get;set;}
 Public  Transient Attachment tAttachment{get;set;}
    public boolean showC1RecordType {get;set;}
    public boolean showC2RecordType {get;set;} 
    
    private Id c1RecordTypeId; 
    private Id c2RecordTypeId;
    
 
    /*
    *   constructor
    */
    public SCOUpload(ApexPages.StandardController controller)
    {
 
        //initialize attachment container
        this.tAttachment = new Attachment();
        c1RecordTypeId = Schema.SObjectType.Purchase_Order__c.getRecordTypeInfosByName().get('C1').getRecordTypeId();
        c2RecordTypeId = Schema.SObjectType.Purchase_Order__c.getRecordTypeInfosByName().get('C2').getRecordTypeId();
        
        purchaseOrder = new Purchase_Order__c();
        purchaseOrder = (Purchase_Order__c)controller.getRecord();
         
        String isButtonClicked = Apexpages.currentPage().getParameters().get('setDefaultValues');
        if(isButtonClicked == 'true'){
            setDefaultValues();
        }
        
        showC1RecordType = false;
        showC2RecordType = false;
        tAttachment= new Attachment();
        
        
        if(purchaseOrder.RecordTypeId == c1RecordTypeId){
            showC1RecordType = true;
            showC2RecordType = false;
        }
        else if(purchaseOrder.RecordTypeId == c2RecordTypeId){
            showC1RecordType = false;
            showC2RecordType = true;
        }
        
    }
    
     public void setDefaultValues(){
        String quoteId = Apexpages.currentPage().getParameters().get('quoteId');
        Quote__c quote = [Select o.id, o.name, o.Quantity_formula__c, o.Opportunity_Product_Detail__r.Opportunity__c,
                    o.Opportunity__c, o.Record_type_name__c,o.Company_Name__c,Opportunity_Product_Detail__r.Opportunity__r.Account__c from Quote__c o WHERE O.id=:quoteId];
        //system.assert(false,quote);     
        purchaseOrder.Opportunity__c = quote.Opportunity_Product_Detail__r.Opportunity__c;
        purchaseOrder.Quote__c = quote.id;
        purchaseOrder.Company__c = quote.Opportunity_Product_Detail__r.Opportunity__r.Account__c;
        
        if(quote.Record_type_name__c == 'C1'){
            purchaseOrder.RecordTypeId = c1RecordTypeId;
        }
        else {
            purchaseOrder.RecordTypeId = c2RecordTypeId;
        }
    }
 
    
 
    /*
    *   upload file
    */
    public PageReference doUploadFile(){
 
        if(tAttachment.Body == null){
            Apexpages.addMessage(new Apexpages.Message(Apexpages.Severity.ERROR, 'Please upload recent Purchase Order  before saving the record'));
            return null;
        }
        else{
            try{
                insert purchaseOrder;
                
              tAttachment.OwnerId = UserInfo.getUserId();
               tAttachment.ParentId = purchaseOrder.id;
              insert tAttachment;
                
                
                return new PageReference('/'+purchaseOrder.id);
            }
            catch(DMLException e){
                ApexPages.addMessage(new ApexPages.message(ApexPages.severity.ERROR,e.getMessage()));
                return null;
            }
        }
    }
}