Warn when defining null handlers empty, fixes #284

Author: Seldaek

DependencyInjection/MonologExtension.php

@@ -726,7 +726,12 @@ private function buildHandler(ContainerBuilder $container, $name, array $handler
             break;
 
         default:
-            throw new \InvalidArgumentException(sprintf('Invalid handler type "%s" given for handler "%s"', $handler['type'], $name));
+            $nullWarning = '';
+            if ($handler['type'] == '') {
+                $nullWarning = ', if you meant to define a null handler in a yaml config, make sure you quote "null" so it does not get converted to a php null';
+            }
+
+            throw new \InvalidArgumentException(sprintf('Invalid handler type "%s" given for handler "%s"' . $nullWarning, $handler['type'], $name));
         }
 
         if (!empty($handler['nested']) && true === $handler['nested']) {