Is WhatsApp is secure?

We're all using WhatsApp.Each of your chats has its own security code used to verify that your calls and the messages you send to that chat are end-to-end encrypted.
We are very happy when we know that our messages are safe,and no o third person can't read our whatsapp messages.A few days ago a research has come to the fore that some experts say that,Whatsapp is not totally secure.In some situations our messages can be read by the third person.Whatsapp did not take any good action for this.Whatsapp says that these are wrong, everything is okay.

When we send a message to someone, a key is needed,and this key is QR CODE

When you send a message to a person, you need to have his public key, i mean his qr code.If you do not have his public key then you can not send him any message.
When you send him a message,Then he will unlock your message with his public key.And he will be able to read that message.

If sometimes there is no Internet connection in this phone or switch are off or the phone is unrechable,at that time you send a message to him Whatsapp shows a single tick in that message.Thats means this message did not delivard to the user.If the user's SIM card is used by someone else Or the number is registered in WhatsApp anyway and then he tells the whatsapp that he is the person whom you sent the message.Then WhatsApp will send that message to the third person.
This opportunity is very low that someone will track you.
If the government wants, then they can read your WhatsApp chat in the same way.

If you follow some steps, your problem will be solved
At 1st go to your WhatsApp setting > Account > Security > and enable your show security notification

This will be the case if the person's security code is changed, then whatsapp will tell you.Then you call him by knowing that he has not changed the security code himself or anyone else has done it.

Thank You @sujoy555