Firebase 'currentUser = some' but 'email = nil' 'username = nil' 'bid = nil'

When I want to upload something to the database but I got fatal error: unexpectedly found a nil while unwrapping an Optional value log. I found that currentUser = some but email, password, uid etc which are inside the currentUser are nil. I also found a EXC_BAD_INSTRUCTION here

func savePost(){
       
        var text: String!
       
        if let postName = postName.text {
            text = postName
        }
       // The EXC_BAD_INSTRUCTION on the line of code below
        let newPost = Posts(postName: text, username: self.currentUser.username, postId: NSUUID().uuidString, postDate: (NSDate().timeIntervalSince1970 as NSNumber), postType: "TEXT", uid: self.currentUser.uid, tutUrl: url)
        let postRef = self.dataBaseRef.child("tuts").childByAutoId()
        postRef.setValue(newPost.toAnyObject(), withCompletionBlock: { (error, ref) in
           / if error == nil {
                self.navigationController!.popToRootViewController(animated: true)
            }else {
                print(error!.localizedDescription)
               
            }*/
        })
           
           
           
           
        }
      

I am using Firebase and Swift. Please help. Thanks