I was correcting some mistakes when I began to receive this error on something that previously worked and I'm not sure why it's not working now. I've narrowed it down to this section of code:

List<Contract_Line_Item__c> current_cls=[select Id, hasCPI__c, Name, Product__c, Net_Price__c, Quantity__c, Maintenance_Amount__c, isCheckbox__c from Contract_Line_Item__c where Contract__c=:current.Id];
    
    List<Contract_Line_Item__c> cls=new  List<Contract_Line_Item__c>();
   
    boolean cpiX;
    for(Contract_Line_Item__c cli:current_cls){
     if(cli.Maintenance_Amount__c > 0)
     {	
     	
     	cpiX=false;
     	Contract_Line_Item__c cl = cli.clone(false, true);
     	Contract_Line_Item__c cpi = cli.clone(false, true);
     	cl.Net_Price__c=0;
     	cl.Services_Amount__c=0;
     	if(!cli.isCheckbox__c)
     	{
     	cpi.Net_Price__c=0;
     	cl.Contract__c=c.Id;
     	cl.Net_Price__c=0;
     	if(current.CIPIX_Increase__c=='Yes' && current.CPI_Increase_Percent__c!=null){
      		if(cli.hasCPI__c)
      			cl.Maintenance_Amount__c*=1+(current.CPI_Increase_Percent__c/100);
     		cl.hasCpi__c=true;
     		cpi.hasCPI__c=true;
     		cpi.Contract__c=c.Id;
     		cpi.Maintenance_Amount__c=cl.Maintenance_Amount__c*(current.CPI_Increase_Percent__c/100);
     		cpi.Name=cli.Name;
     		cpi.isCheckbox__c=true;
     		cpiX=true;
     		}
     	cls.add(cl);
     	if(cpiX)
     		cls.add(cpi);
     	
     	a=new Asset();
     	a.AccountId = c.AccountId;
  	    a.Product2Id=cli.Product__c;
   	    a.Name=cli.Name + ' - Asset';
 	    a.ContactId=Cont;
 	    if(c.startDate!=null){
 	    a.installDate=c.StartDate;
 	    date enddate=c.StartDate.addMonths(12).addDays(-1);
 	    a.UsageEndDate=enddate;
 	    }
 	    
 	    insert a;}
 	    else{
 	    	delete cli;}
     	
	  }
    }
   
    insert cls;

And when I comment out the "If.isCheckBox__c) sections, I get a 'de-referencing a null object error'. Any advice?