Notice: Undefined index: pagenow

I found a minor bug that is easy to resolve. There is a missing test for existence and value present of the $GLOBALS['pagenow'] property.

It manifests by a PHP Notice:

> Notice: Undefined index: pagenow in /wp-content/plugins/filter-page-by-template/filter-page-by-template.php on line 17

## Solution

- L17: if( $GLOBALS['pagenow'] == 'edit.php' ) {
+ L17: if( !empty($GLOBALS['pagenow']) && $GLOBALS['pagenow'] == 'edit.php' ) {

I’d submit a PR if I could find a Github repo for the plugin.