I am using feild sets in this program but error occcure in this program and pls give suggestion

public class CreateMultiplePatients{
    ID DoctorID;
    public Doctor__c docObj{get;set;}
    
    public List<Patient__c> multiplePatients{get;set;}
    
    public CreateMultiplePatients(ApexPages.StandardController varName){
        multiplePatients = New List<Patient__c>();
        
        DoctorID = apexpages.currentpage().getparameters().get('id');
        
      //  docObj = [SELECT id,name,daysworking__c,Joining_Year__c,Salary__c,specialization__c FROM Doctor__c where id=:DoctorID];      
     
     
     
        String validateQuery = 'SELECT id';
        for(Schema.FieldSetMember fld :SObjectType.Doctor__c .FieldSets.Doctor_feild.getFields()) {
                 validateQuery += ', ' + fld.getFieldPath();
        }
        validateQuery += ' from Doctor__c WHERE id=:DoctorID';
        
        docObj = dataBase.Query(validateQuery);
     
     
        Patient__c newpatient = New Patient__c();
        multiplePatients.add(newpatient);
    
    }
    
    public void addRow(){
         Patient__c newpatient = New Patient__c();
         multiplePatients.add(newpatient);
    }
    
    public pageReference saveMultiplepatients(){
        for(integer i=0;i<multiplePatients.size();i++){
           multiplePatients[i].TreatmentDoctor__c = DoctorID;
          //ApexPages.addMessage(new ApexPages.Message(ApexPages.Severity.ERROR,'Error Message.'));

        }
        insert multiplePatients;
        
        pageReference pagref = New pageReference('/'+DoctorID);
        return pagref;       
    }    
 public void RemoveRow(){
        integer i = multiplePatients.size();
        multiplePatients.remove(i-1);
    }


}