Fix error 500 on feed update when feed ref ID is missing

lib/Feed.php

@@ -302,11 +302,14 @@ class Feed extends PhpObject {
 	}
 
 	private function retrieveFeed() {
-		$sContent = '[]';
-		if($this->sRefFeedId !='') {
+		if($this->sRefFeedId == '') {
+			$sContent = '{"response":{"errors":{"error":{"code":"","text":"Feed reference ID missing","description":"Feed reference ID missing"}}}}';
+		}
+		else {
 			$sUrl = self::FEED_HOOK.$this->sRefFeedId.self::FEED_TYPE_JSON;
 			$sContent = file_get_contents($sUrl);
 		}
+
 		return json_decode($sContent, true);
 	}