Fix issue identified by Psalm

Author: sebastianbergmann

.psalm/baseline.xml

@@ -49,9 +49,6 @@
     <ArgumentTypeCoercion>
       <code><![CDATA[$eventClass . 'Subscriber']]></code>
     </ArgumentTypeCoercion>
-    <PossiblyNullReference>
-      <code>flush</code>
-    </PossiblyNullReference>
   </file>
   <file src="src/Event/Value/Runtime/PHP.php">
     <ImpureFunctionCall>

src/Event/Facade.php

@@ -100,7 +100,7 @@ public static function forward(EventCollection $events): void
 
     public static function seal(): void
     {
-        self::$deferringDispatcher->flush();
+        self::deferredDispatcher()->flush();
 
         self::$sealed = true;