[SalesForce] Test class for a controller – Unable to set the variable in the constructor

I have a controller:

public class XYZcontroller{
    public object__c sr{get;set;}
    public String srid;
    public XYZcontroller(ApexPages.StandardController controller) {
        sr = (object__c) controller.getRecord();
        srid = String.valueOf(sr.Id).substring(0, 15);
        String pageMessage = ApexPages.currentPage().getParameters().get('pageMessage');
        boolean doNotDisplay;
        String referrer = ApexPages.currentPage().getHeaders().get('REFERER');
        if(referrer.contains(srid+ '/e')){
            doNotDisplay=false;
        }else{
            doNotDisplay=true;
        }
    }
}

Test class for the above:

@isTest
public class xyzControllerTest{        
    static testMethod void testMethod1(){        
        object__c sr = new object__c (Name='test');
        insert sr;
        PageReference pageRef = Page.Thepage;
        Test.setCurrentPage(pageRef);
        ApexPages.currentPage().getParameters().put('pageMessage', 'Amessage');     
        ApexPages.currentPage().getHeaders().put('REFERER', sr.id + '/e');
        ApexPages.StandardController stdcontroller = new ApexPages.StandardController(sr);

        xyzController controller = new xyzController(stdcontroller);     
    }    
}

In the above test class I get an error at line – xyzController controller = new xyzController(stdcontroller); saying:

Error Message:

System.NullPointerException: Argument 1 cannot be null Stack Trace External entry point Class.xyzControllerTest.testMethod1: line 14, column 1

I am thinking it is due to the public String srid; in the XYZcontroller which does not get a value inside the constructor. ( srid = String.valueOf(sr.Id).substring(0, 15); )