I Have Database Errors on my Codeigniter script - php

Question

i install a school management system and one time login but after logout and enter username and password show a box with this message:
A Database Error Occurred , You have an error in your SQL syntax; check the manual that corresponds to your MySQL server version for the right syntax to use near 'WHERE _id IS NULL' at line 2 . SELECT * WHERE _id IS NULL . Filename: views/backend/header.php . Line Number: 34 "
header.php code :
<div class="row">
<div class="col-md-12 col-sm-12 clearfix" style="text-align:center;">
<h2 style="font-weight:200; margin:0px;"><?php echo $system_name;?></h2>
</div>
<!-- Raw Links -->
<div class="col-md-12 col-sm-12 clearfix ">
<ul class="list-inline links-list pull-left">
<!-- Language Selector -->
<div id="session_static">
<li>
<h4>
<a href="#" style="color: #696969;"
<?php if($account_type == 'admin'):?>
onclick="get_session_changer()"
<?php endif;?>>
<?php echo get_phrase('running_session');?> : <?php echo $running_year.' ';?><i class="entypo-down-dir"></i>
</a>
</h4>
</li>
</div>
</ul>
<ul class="list-inline links-list pull-right">
<li class="dropdown language-selector">
<a href="<?php echo site_url('home');?>" target="_blank">
<i class="entypo-globe"></i> Website
</a>
<a href="#" class="dropdown-toggle" data-toggle="dropdown" data-close-others="true">
<i class="entypo-user"></i>
<?php
$name = $this->db->get_where($this->session->userdata('login_type'), array($this->session->userdata('login_type').'_id' => $this->session->userdata('login_user_id')))->row()->name;
echo $name;
?>
</a>
<?php if ($account_type != 'parent'):?>
<ul class="dropdown-menu <?php if ($text_align == 'right-to-left') echo 'pull-right'; else echo 'pull-left';?>">
<li>
<a href="<?php echo site_url($account_type . '/manage_profile');?>">
<i class="entypo-info"></i>
<span><?php echo get_phrase('edit_profile');?></span>
</a>
</li>
<li>
<a href="<?php echo site_url($account_type . '/manage_profile');?>">
<i class="entypo-key"></i>
<span><?php echo get_phrase('change_password');?></span>
</a>
</li>
</ul>
<?php endif;?>
<?php if ($account_type == 'parent'):?>
<ul class="dropdown-menu <?php if ($text_align == 'right-to-left') echo 'pull-right'; else echo 'pull-left';?>">
<li>
<a href="<?php echo site_url('parents/manage_profile');?>">
<i class="entypo-info"></i>
<span><?php echo get_phrase('edit_profile');?></span>
</a>
</li>
<li>
<a href="<?php echo site_url('parents/manage_profile');?>">
<i class="entypo-key"></i>
<span><?php echo get_phrase('change_password');?></span>
</a>
</li>
</ul>
<?php endif;?>
</li>
<li>
<a href="<?php echo site_url('login/logout');?>">
<?php echo get_phrase('log_out'); ?><i class="entypo-logout right"></i>
</a>
</li>
</ul>
</div>
</div>
<hr style="margin-top:0px;" />
<script type="text/javascript">
function get_session_changer()
{
$.ajax({
url: '<?php echo site_url('admin/get_session_changer');?>',
success: function(response)
{
jQuery('#session_static').html(response);
}
});
}
</script>
line 34 :
$name = $this->db->get_where($this->session->userdata('login_type'), array($this->session->userdata('login_type').'_id' => $this->session->userdata('login_user_id')))->row()->name;
I am beginner . please help me step by step