警告:mysqli::query(): could 't fetch mysqli in.我怎么解决它呢?

我有这个代码，我得到这些错误:

警告:mysqli::query(): cannot fetch mysqli in C:'xampp'htdocs'pliroforiaka'grammateiakos_upallilos.php on line 299

注意:试图在C:'xampp'htdocs'pliroforiaka'grammateiakos_upallilos.php中获得非对象的属性

警告:mysqli::close():无法在C:'xampp'htdocs'pliroforiaka'grammateiakos_upallilos.php中获取mysqli

<table name="table5" id="table5" width="941" border="1" cellspacing="0" cellpadding="3" align="center">
<form name="form5" class="form5" id="form5" action="stoixeiarantevou.php" method="post"> 
<td>
<?php
$sql = "SELECT username FROM giatros";
$result = $conn->query($sql);
if ($result->num_rows > 0) {
?>
<div id="doc">
<select name="Doctor">
<?php
// output data of each row
while($row = $result->fetch_assoc()) {
?>
    <option value="<?php echo $row['Doctor']; ?>"><?php echo $row['username'];?>        </option>
<?php
}
?>
</select>
</div>
<?php
 }
$conn->close();
   ?>
 Ημερομηνία: <input type="text" id="Hmeromhnia" name="Hmeromhnia" required>   <br>
 Ώρα: <input type="text" id="Wra" name="Wra" required><br>
 <input type="button" value="Δημιουργία" id="btnsubmit" onclick="submitForm3()" required>
 </td>
 </form>
 </table>
 <script>
  function submitForm3() {
   // Get the first form with the name
   // Hopefully there is only one, but there are more, select the correct index
   var frm = document.getElementsByName('form5')[0];
   frm.submit(); // Submit
   frm.reset();  // Reset
   alert("Τα στοιχεία σας ανανεώθηκαν επιτυχώς");
   return false; // Prevent page refresh
     }
   </script>
   </td>
   </tr>