Manul Sharing The Record Throw Apex code in Trigger...........

trigger ShareRecordWithIM on nnr__c (after update)
{
    List<nnr__Share> shareList = new List<nnr__Share>();
    List<nnr__Share> deleteList = new List<nnr__Share>();
    nnr__Share ImpShare,ImpDelete;
    List<RecordType> MMRT = [SELECT Id FROM RecordType WHERE DeveloperName IN ('knr','wg')];
    Map<Id,Id> MMRTMap = new Map<Id,Id>{};
    String[] toAddress = new String[]{UserInfo.getUserEmail()};
    List<Messaging.SingleEmailMessage> mails = new List<Messaging.SingleEmailMessage>();
    Messaging.SingleEmailMessage email;
    Id oldRecShare;
   
    for(RecordType rt : MMRT)
        MMRTMap.put(rt.Id,rt.Id);
        
    for(nnr__c newRec : Trigger.new)
    {
        oldRecShare = trigger.oldMap.get(newRec.Id).Share_Record__c;
        if(newRec.Share_Record__c !=null && (oldRecShare != newRec.Share_Record__c) && MMRTMap.containsKey(newRec.RecordTypeId))
        {
           ImpShare = new nnr__Share();
           ImpShare.ParentId = newRec.Id;
           ImpShare.UserOrGroupId = newRec.Share_Record__c;
           ImpShare.AccessLevel = 'Edit';
           shareList.add(ImpShare);
          
           //send email to TL
            email = new Messaging.SingleEmailMessage();
            email.setToAddresses(toAddress);
            String IM = [SELECT Name FROM User WHERE Id =:newRec.Share_Record__c LIMIT 1].Name;
            email.setSubject(newRec.Opportunity_UniqueID__c+' Case Access Shared Notification');
            email.setHTMLBody('Hello,<br><br>Shared access to Implementation '+newRec.Name+' has been given to '+IM+'.<br><br>Kind regards,<br>Centralised Implementation Solutions '+newRec.Market_Referred__c);
            email.setSaveAsActivity(false);
            mails.add(email);
        }
        if((oldRecShare != null) && (oldRecShare != newRec.Share_Record__c) && MMRTMap.containsKey(newRec.RecordTypeId))
        {
            ImpDelete = [SELECT Id FROM nnr__Share WHERE UserOrGroupId =:oldRecShare AND ParentId =: newRec.Id];
            deleteList.add(ImpDelete);
        }  
    }
   
    try
    {
        if(shareList.size()>0)
            insert shareList;
        if(deleteList.size()>0)
            delete deleteList;
        if(mails != null)   
            //Messaging.sendEmail(new Messaging.SingleEmailMessage[] {email});
            Messaging.sendEmail(mails);
    }
    catch(Exception e)
    {
        System.debug('Following error occured while inserting sharing record: '+e);
    }
}


============================TEST CLASS=======================

@isTest

public class Test_ShareRecordWithIM
{
    static TestMethod void testSRIM()
    {       
        Profile p = [select id from profile where name='System Administrator'];   
       
        //User Insert
        User u = new user(alias = 'stdt', email='testsharerecorduser1@testorg.com', emailencodingkey='UTF-8', lastname='DPSRTest', languagelocalekey='en_US', localesidkey='en_US', profileid = p.Id, timezonesidkey='America/Los_Angeles', username='testsharerecorduser1@testorg.com',Segment__c = 'Mid Market', Market__c = 'UK');
        Insert u;
       
        User u1 = new user(alias = 'stdt', email='test_share_record_user_2@testorg.com', emailencodingkey='UTF-8', lastname='DPSRTest', languagelocalekey='en_US', localesidkey='en_US', profileid = p.Id, timezonesidkey='America/Los_Angeles', username='test_share_record_user_2@testorg.com',Segment__c = 'Mid Market', Market__c = 'UK');
        Insert u1;
       
       
        //Account Insert
       
        Account acc = new Account();
        acc.Name = 'Test Account';
        acc.AnnualRevenue = 12345;
        acc.CurrencyIsoCode = 'USD';
       
        Insert acc;
        test.starttest();   
        //Opportunity Record 1
        Opportunity opp = new Opportunity();
        opp.Name = 'Test Opp';
        opp.StageName = '7 - Contract Fully Executed';
        opp.Core_Product__c = 'Corporate Card';
        opp.Sub_Product__c = 'Gold';
        opp.Type = 'New';
        opp.CloseDate = System.now().date();
        opp.Opportunity_Market__c = 'UK';
        opp.AccountId = acc.Id;
        opp.LeadSource = 'Test';
        opp.Probability = 34;
        opp.Potential_CV_USD__c = 23;
        opp.CurrencyIsoCode = 'USD';
        opp.Commodity__c = 'Test';
       
        Insert opp;
       
        EMEA_Implementation__c imp = new EMEA_Implementation__c();
        imp.Name = 'Test imp';
        imp.Opportunity__c = opp.Id;
       
        Insert imp;
        
        imp.Share_record__c = u.Id;
       
        Update imp;
       
        imp.Share_record__c = u1.Id;
       
        Update imp;
        Test.stoptest();
    }