Single Email Message Error -- REQUIRED_FIELD_MISSING, Missing target address

I have written a trigger in which I want to send an email to the Email address of Contact. But when i am testing the trigger it is giving me an error of

REQUIRED_FIELD_MISSING, Missing target address (target, to, cc, bcc): []:

Following is the code for trigger

trigger SendEmailMessageToFlatContact on Building_Wing__c (after update) {

    set<id> setValidBuildId = new set<id>();
    List<Flat__c> listFlat = new List<Flat__c>();
    List<String> ListOkEmails = new List<String>();
    List<String> ListPaymentEmails = new List<String>();
    map<String, Flat__c> mapEmailFlat = new map<String, Flat__c>();
    boolean Flag;

    for(Building_Wing__c objBuild: trigger.new)
    {
        if(trigger.oldMap.get(objBuild.id).Stage_Completed__c != trigger.newMap.get(objBuild.id).Stage_Completed__c)
        {
            setValidBuildId.add(objbuild.id);
        }
    }

    listFlat = [Select id, name, Primary_Contact__c,Building_Wing__c,Amount_Due_By_Stage__c,Building_Wing__r.Stage_Completed__c ,Primary_Email__c,Contact_Phone__c, Flat_Total_Cost__c, Amount_Received__c  from flat__c where Building_Wing__c In: setValidBuildId and Flat_Total_Cost__c >0];
    Decimal PaidPercentage = 0;
    String body;
    List<Messaging.SingleEmailMessage> mails = new List<Messaging.SingleEmailMessage>();
    EmailTemplate templateId = [Select id from EmailTemplate where DeveloperName= 'Installment_Due_Template'];    
    List<ID> SendTargetObject =  new List<ID>();
    for(flat__c objFlat: listFlat)
    {          
        PaidPercentage = (objFlat.Amount_Received__c / objFlat.Flat_Total_Cost__c)*100;
        Messaging.SingleEmailMessage mail =  new Messaging.SingleEmailMessage();

        if(PaidPercentage < objFlat.Building_Wing__r.Stage_Completed__c && objFlat.Primary_Contact__c != null && objFlat.Primary_Email__c != null && objFlat.Contact_Phone__c != null)
        {

            ListPaymentEmails.add(objFlat.Primary_Email__c);
            body = 'Dear Customer Please Pay Pending Amount::'+objFlat.Amount_Due_By_Stage__c;
            SMSUtil.sendSms(objFlat.Contact_Phone__c,body);
            mapEmailFlat.put(objFlat.Primary_Email__c, objFlat);
            mail.setToAddresses(ListPaymentEmails);
            mail.setTargetObjectId(objFlat.Primary_Contact__c);
            mail.setTemplateID(templateId.Id); 
            mail.setSaveAsActivity(true);
            system.debug('IF Mail-------------->'+mail);

        }
        else if(objFlat.Primary_Contact__c != null && objFlat.Primary_Email__c != null)
        {
            body = 'Dear Customer New Stage has been Completed.';
            SMSUtil.sendSms(objFlat.Contact_Phone__c,body);
            ListOkEmails.add(objFlat.Primary_Email__c);
            mail.setToAddresses(ListPaymentEmails); 
            mail.setTargetObjectId(objFlat.Primary_Contact__c);
            mail.setTemplateID(templateId.Id); 
            mail.setSaveAsActivity(true);
           system.debug('else Mail-------------->'+mail);
        }

        mails.add(mail);   
    }
    system.debug('ListPaymentEmails >>' +ListPaymentEmails + 'ListOkEmails >>' +ListOkEmails);
    // Email message to both group
    system.debug('Mails------------->'+mails);

      if(mails.size() > 0){
          Messaging.sendEmail(mails);
      }

}