fatal error: unexpectedly found nil while unwrapping an Optional value ( before: "Hello Firebase", after: textView.text

Question

I'm making a database with firebase.

@IBAction func addPost(_ sender: Any) {

    // Post the data to firebase
    ref?.child("Posts").childByAutoId().setValue("Hello Firebase")

    // Dismiss the popover
    presentingViewController?.dismiss(animated: true, completion: nil)
}

The above code is working correctly and like this

and I change it to the code below:

@IBAction func addPost(_ sender: Any) {

    // Post the data to firebase
    ref?.child("Posts").childByAutoId().setValue(textView.text)

    // Dismiss the popover
    presentingViewController?.dismiss(animated: true, completion: nil)
}

after changing it I get:

"fatal error: unexpectedly found nil while unwrapping an Optional value" error occurred....

How can I fix it?