I have eventsbooking installed and when we go to the registration page of an event we are getting this error

Internal Server Error
Internal Server Error
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
<html xmlns="http://www.w3.org/1999/xhtml" xml:lang="en-gb" lang="en-gb" dir="ltr">
<head>
	<title>0 - Error: 0</title>
	<meta http-equiv="Content-type" content="text/html; charset=utf-8" />
	<meta name="viewport" content="width=device-width, initial-scale=1.0">

	<link href="https://fonts.googleapis.com/css?family=Archivo:400,500,700&display=swap" rel="stylesheet">

	<link rel="stylesheet" href="/templates/ja_alpha/css/error.css" type="text/css" />

</head>
<body class="page-error">
	<div class="main">
		<div class="error">
			<div id="outline">
				<div id="errorboxoutline">
					<div class="error-code"><span class="lastfirst">0</span>					</div>
					<div class="wrap-text">
						<div class="error-message"><h2><span>Call to a member function get() on null</span></h2></div>
						<div id="errorboxbody">
							<p>Please try one of the following pages:</p>
						</div>
						<a class="button-home" href="/index.php" title="Go to the Home Page">Home Page<span class="ion-android-arrow-forward"></span></a>
					</div>
				</div>
			</div>
		</div>
	</div>
</body>
</html>

This doesn't seem to be an error with the events software, but I have asked the developer there too.
Can you help us to fix this please?

The page is https://avonlca.org.uk/events/upcoming-events/planning-in-plain-english/group-registration

Thanks
Shemeam