Fatal error: Call to a member function prepare() on null problem here

Question

Fatal error: Call to a member function prepare() on null

public function __construct(){
    require_once('DbConnect.php');
    $db = new DbConnect();
    $this->dbConn=$db->connect();
}


public function insert(){   
    $sql="INSERT INTO $this->tableName VALUES(null,:name,:type,:pages,:price,:author,:cdate)";
    $stmt=$this->dbConn->prepare($sql);
    $stmt->bindParam(':name',$this->name);
    $stmt->bindParam(':type',$this->type);
    $stmt->bindParam(':pages',$this->pages);
    $stmt->bindParam(':price',$this->price);
    $stmt->bindParam(':author',$this->author);
    $stmt->bindParam(':cdate',$this->createdDate);

    if($stmt->execute()){   
        return true;
    }else{  
        return false;   
    }