Trying to get property of non-object - PHP - php

Question

So I'm trying to get the teacher_id that is corresponding to the teacher's first and last name that the user has entered, but when I try to get the teacher_id it outputs Trying to get property of non-object. Does anyone have any ideas?
PHP
<?php
// PROCESSES STUDENT INFO
// get connect page
require '../../connect.php';
// get input info
$student_id = $_POST['student_id'];
$first_name = $_POST['first_name'];
$last_name = $_POST['last_name'];
$teacher_first_name = $_POST['teacher_first_name'];
$teacher_last_name = $_POST['teacher_last_name'];
// check if input is not empy
if(!empty($student_id) && !empty($first_name) && !empty($last_name) && !empty($teacher_first_name) && !empty($teacher_last_name)) {
// check if numeric inputs have a number
if(is_numeric($student_id)) {
$teacher_check = mysqli_query($link, "SELECT teacher_id FROM teachers WHERE first_name='$teacher_first_name' AND last_name='$teacher_last_name'");
// check if teacher exists
if($teacher_check) {
$row = $teacher_check->fetch_object();
$result = mysqli_query($link, "INSERT INTO students (student_id, first_name, last_name, teacher_id) VALUES ($student_id, '$first_name','$last_name', $row->teacher_id)");
if($result) {
header("Location: ../../../admin.php?message=Success!");
} else {
echo mysqli_error($link);
// header("Location: ../../../admin.php?message=Sorry we ran into an error");
}
} else {
header("Location: ../../../admin.php?message=Teacher Does Not Exist");
}
} else {
header("Location: ../../../admin.php?message=Please add a number for Student ID");
}
} else if (empty($student_id) || empty($first_name) || empty($last_name)) {
header("Location: ../../../admin.php?message=Please add you're input values");
}
?>

Answer 1

change this line, you are checking whether the query is ok but you have not checked whether it has any results.
if($teacher_check) {
$row = $teacher_check->fetch_object();
to (this line checks whether you have any result data if you have result data you have it $row otherwise null)
if($row = $teacher_check->fetch_object()){