title: Only safe or non null assserted kotlin message
body: <p>I'm trying to run an app on mine phone and i'm keep getting this problem with my project pledase help me with this 
  thank youOnly safe or non null assserted calls are allowed on a nullable receiver type of arraylis</p>
  
  <pre><code>register_btn.setOnClickListener { val email = 
  email_registersscreen.text.toString()
     val password = password_registerscreeb.text.toString()
  
     Log.d("MainActiviy", "Email is:" + email)
     Log.d("MainActivity", "Password:$password")
  
     //Firebase Authentication  user  with email and password
  
     FirebaseAuth.getInstance().createUserWithEmailAndPassword(email, password)
         .addOnCompleteListener {
             if (!it.isSuccessful) return@addOnCompleteListener
  
             // else it successful
             Log.d("Main", "successfully created user with uid: 
  ${it.result.user.uid}")
  
             }
     }
  </code></pre>