WSDL generated Apex Class output string

Question

im having trouble getting the output from my webservice which was based in an wsdl generated apex class, it brings through a simple string 'D2978' which will need to be added to a new custom field called invoiceNumber__c everytime a commandbutton is pressed however im getting "attempted to de-reference null object

 public String getInvoiceNumber(String invoicenumberid) {
        ws_GenerateInvoiceNumber.invoicenumberServiceSearchQuery_element request_x = new ws_GenerateInvoiceNumber.invoicenumberServiceSearchQuery_element();
        request_x.invoicenumberid = invoicenumberid;
        ws_GenerateInvoiceNumber.invoicenumberServiceResponse_element response_x;
        Map<String, ws_GenerateInvoiceNumber.invoicenumberServiceResponse_element> response_map_x = new Map<String, ws_GenerateInvoiceNumber.invoicenumberServiceResponse_element>();
        response_map_x.put('response_x', response_x);
        WebServiceCallout.invoke(
          this,
          request_x,
          response_map_x,
          new String[]{endpoint_x,

im then referencing this in an apex class to move the string to the custom field however this is where i think im going wrong any pointers would be huge help getting very frustrated at this

        private static string callWebservice(String invoicenumberid ){
        ws_GenerateInvoiceNumber.InvoiceNumberSearchServicePort soapService = new ws_GenerateInvoiceNumber.InvoiceNumberSearchServicePort();
        string response = soapService.getInvoiceNumber( invoicenumberid);
    return response;
}

    private static string callWebserviceUpdate(String invoicenumberid ){
        ws_GenerateInvoiceNumber.InvoiceNumberSearchServicePort soapService = new ws_GenerateInvoiceNumber.InvoiceNumberSearchServicePort();
        string response = soapService.incrementupdateInvoiceNumber(invoicenumberid);
    return response;
}

    public void InvoiceNumberView (String projectId) {
        this.Project = [SELECT Id, Name, InvoiceNumber__c FROM Project__c WHERE Id =: projectId];

        // callWebservice(String invoicenumberid);
        // callWebservice(string response);
         string response = Project.InvoiceNumber__c ;
        // string response =  callWebservice(String.valueof(this.project.invoicenumber__c));
        // string response;
        // Project.InvoiceNumber__c = response;


    update project;
}

Answer 1

Where you catch the exception, use the getLineNumber() and getStackTraceString() methods to determine the location of the NullPointerException.

Then examine the line in question to find the variables that could possibly be null.

If there are more than one, use something like the following immediately before the line that threw the exception.

System.assertNotEquals(null, variableThatMightBeNull, 'variableThatMightBeNull was the issue!');

Answer 2

    public vf_projectComplete(){
    this.project = [SELECT Id, Name, InvoiceNumber__c, updateBoolean__c  
                    FROM Project__c
                    WHERE Id =: projectId];
    GINC = new GenerateInvoiceNumberController(this.project.Id);
    this.productOrders = [SELECT Id, Name, Product__r.DisplayName__c, Complete__c 
                          FROM ProductOrder__c 
                          WHERE Project__c =: this.projectId];
}

i forgot to add the controller to the constructor this then found an error which was sorted in my wsdl