Fix undefined property warning in Item::__call magic method

Add property_exists() checks before accessing dynamic properties in
get, has, and add methods to prevent PHP warnings when methods like
add_category() are called for properties that don't exist on the class.

Fixes #541

Author: pfefferle

includes/Entity/class-item.php

@@ -115,19 +115,19 @@ public function __call( $method, $params ) {
 		$var = strtolower( substr( $method, 4 ) );
 
 		if ( strncasecmp( $method, 'get', 3 ) === 0 ) {
-			return $this->$var;
+			return property_exists( $this, $var ) ? $this->$var : null;
 		}
 
 		if ( strncasecmp( $method, 'has', 3 ) === 0 ) {
-			return ! empty( $this->$var );
+			return property_exists( $this, $var ) && ! empty( $this->$var );
 		}
 
 		if ( strncasecmp( $method, 'set', 3 ) === 0 ) {
 			$this->$var = current( $params );
 		}
 
 		if ( strncasecmp( $method, 'add', 3 ) === 0 ) {
-			if ( ! $this->$var ) {
+			if ( ! property_exists( $this, $var ) || ! $this->$var ) {
 				call_user_func( array( $this, 'set_' . $var ), current( $params ) );
 				return true;
 			}

readme.md

@@ -101,6 +101,10 @@ While not all display options can be settings, we are looking to provide some si
 
 Project and support maintained on github at [pfefferle/wordpress-webmention](https://github.com/pfefferle/wordpress-webmention).
 
+### 5.5.1
+
+* Fix undefined property warning in `Item::__call` magic method when accessing non-existent properties like `category`.
+
 ### 5.5.0
 
 * Fix `webmentions_disabled` meta value not being deleted.