Blank page when clicking preview changes

Hi guys,

I have a problem with the WordPress page preview as it doesn’t load the preview page but instead in redirects to a blank page with the link:
https://abcd.com/wp-admin/post.php

I checked the similar topics but I couldn’t find a solution yet.
I went to the error_log and apparently there is an error with a plugin I am using called ninja forms:

PHP Warning: session_start(): Cannot send session cache limiter – headers already sent (output started at /home/medro/public_html/wp-content/themes/Avada2/admin/functions/functions.options.php:1) in /home/medro/public_html/wp-content/plugins/ninja-forms/ninja-forms.php on line 294

I went in that directory and at the 294th line (the bold one) shows this:

function ninja_forms_set_transient_id(){
if( !session_id() )
session_start();
if ( !isset ( $_SESSION[‘ninja_forms_transient_id’] ) AND !is_admin() ) {
$t_id = ninja_forms_random_string();
// Make sure that our transient ID isn’t currently in use.
while ( get_transient( $t_id ) !== false ) {
$_id = ninja_forms_random_string();
}
$_SESSION[‘ninja_forms_transient_id’] = $t_id;
}
}

I had this problem before updating to the latest version of WordPress.
I would be very grateful if somebody solves it as I am making good use of this plugin. Thank you!