I am trying to write a test for a visualforce page where user is searching for a keyword to choose a record to add to the opportunity. How do I solve my error?

error

Test Class

@IsTest public class TestAddCustomerProductStage1create {

    /* This is a basic test which simulates the primary positive case for the 
       save method in the AddCustomerProduct class. */
Static AddCustomerProductStage1create ext;
public static testMethod void myUnitTest() {
    
    Account acc1=new Account(Name='test', BillingStreet='test', BillingCity='NY', BillingState='test',
                BillingCountry='US', BillingPostalCode='test', ShippingStreet='test', ShippingCity='test',
                ShippingState='NY', ShippingCountry='US', ShippingPostalCode='test');
insert acc1;                


Opportunity op = new Opportunity(Name='TestOppty',closedate=date.parse('1/1/2222'),
                    stagename = 'prospecting',AccountID=acc1.id,Equip_Services_Options_to_be_quoted__c='test', 
                    Opportunity_Region__c='test');
insert op;   

    System.assertEquals('TestOppty', op.name);
            
    PageReference pageRef = Page.CustomerProductStage1create;
    
    pageRef.getParameters().put('oppid', op.id);

    Test.setCurrentPageReference(pageRef);
    
    Customer_Product_Line_Item__c c = new Customer_Product_Line_Item__c(name='Apple', opportunity__c=op.id);
    insert c;
    
    Customer_Product__c cp = new Customer_Product__c(name='Apple');
    
    System.assert (true, [select name from Customer_Product_Line_Item__c where name = 'Apple']);

               ApexPages.StandardController sc = new ApexPages.StandardController(New Customer_Product_Line_Item__c());     
               
                ext = new AddCustomerProductStage1create(sc );           
    
     String cpUrl = '/apex/customerproductstage1create?oppid=' + op.id;
     
        System.Debug('++++++++++++'+cpUrl);
    
     PageReference ref = new PageReference(cpUrl);
          
     Test.setCurrentPage(ref); 
     
     ApexPages.currentPage().getParameters().put(cp.name, 'ap');
     
     PageReference proc = ext.processSelected();
 
}
}

 Class

public class AddCustomerProductStage1create{

/* Constructor Function. The CustomerProduct id is captured in this function */

public Customer_Product_Line_Item__c cpl {get;set;}
public Opportunity o;
public AddCustomerProductStage1create(ApexPages.StandardController c)
{
            o = [select id from Opportunity where id =
                       :ApexPages.currentPage().getParameters().get('oppid')];
}

      public Opportunity getOpportunity() {
            return o;
      }

/* Variable declarations */

public List<cCustomer> cpList {get; set;}                              // Wrapper class which stores customer product data and a boolean flag.
public List<cCustomer> cpsel{get; set;}
public ID oid {get; set;}

String userinput;                                                               // cp Name
String userinp;  



Public List<Customer_Product__c> results = new List<Customer_Product__c>();                 // Customer_Product__c search results.

Public List<Customer_Product__c> selproduct = new List<Customer_Product__c>();                 // Customer_Product__c selectedproduct.

/* End of Variable declarations */

/* Getter and setter methods for getting the user input ie. Customer_Product__c name from the UI */

public String getuserinput(){return userinput;}
public void setuserinput(String userinp)
{

this.userinput=userinp;
system.debug('*****SFDC-TEST-1**********'+userinput+'='+userinp);

}
public String getselproduct(){return null;}

/*End of Getter and Setter methods */

/* Method to Search the Customer_Product__c database to return the query results */
public List<Customer_Product__c> cpsearch()
{
     cpList = new List<cCustomer>();
     for(Customer_Product__c c : [select id, name from Customer_Product__c where Name like :userinput+'%'order by Name asc])
     {
         cpList.add(new cCustomer(c));
        
     }
system.debug('*****SFDC-TEST-2**********'+cpList);
 return null;
}
/* End of method */


/* Method for returning the Customer Product search results to the UI */
public List<cCustomer> getresults(){
    
 return cpList;

}
    public PageReference processSelected() {

                //We create a new list of Customer Products that we be populated only with Customer Products if they are selected
        List<Customer_Product__c> selectedProduct = new List<Customer_Product__c>();

        //We will cycle through our list of cCustomer and will check to see if the selected property is set to true, 
        //if it is we add the Customer Product to the selectedProduct list
        for(cCustomer cCon : getresults()) {
            if(cCon.selected == true) {
                selectedProduct.add(cCon.con);
                
                
            }
        }
system.debug('*****SFDC-TEST-3**********'+selectedProduct);
        // Now we have our list of selected products and can perform any type of logic we want
        System.debug('These are the selected Products...');
        for(Customer_Product__c cCon : SelectedProduct ) {
            system.debug(cCon);
            
            selproduct.add(cCon);
            
        }
         system.debug('*****SFDC-TEST-4**********'+selproduct.size());

    cpsel = new List<cCustomer>();
     for(Customer_Product__c p : [select name from Customer_Product__c where id =:selproduct])
     {
     system.debug('*****SFDC-TEST-p**********'+p.name);
       cpsel.add(new cCustomer(p));
       
        Customer_Product_Line_Item__c cpli = new Customer_Product_Line_Item__c(Name = cpsel[0].con.name, 
                Opportunity__c = o.id);
      
      insert cpli;
      system.debug('*****test cpli.name**********'+cpli.name);     
     }
         PageReference home = new PageReference('/' + o.id);
        home.setRedirect(true);
        return home;

  system.debug('*****SFDC-TEST-5**********'+cpsel.size());
        return null;
    }


/* Wrapper class to contain contact record and a boolean flag */
public class cCustomer{
 public Customer_Product__c con {get; set;}
 public Boolean selected {get; set;}

 /*This is the contructor method. When we create a new cContact object we pass a
 Contact that is set to the con property. We also set the selected value to false*/
 public cCustomer (Customer_Product__c c)
 {
     con = c;
     selected = false;
 }
}

/* end of Wrapper class */    

}

 How do I solve this test coverage error?

Thank you