fix: null check in ListenerCollection notify method (#1661)

Author: kkocel

playwright/src/main/java/com/microsoft/playwright/impl/ListenerCollection.java

@@ -17,6 +17,7 @@
 package com.microsoft.playwright.impl;
 
 import com.google.gson.JsonObject;
+import com.microsoft.playwright.PlaywrightException;
 
 import java.util.*;
 import java.util.function.Consumer;
@@ -46,6 +47,9 @@ <T> void notify(EventType eventType, T param) {
   }
 
   void add(EventType type, Consumer<?> listener) {
+    if (listener == null) {
+      throw new PlaywrightException("Can't add a null listener");
+    }
     List<Consumer<?>> list = listeners.get(type);
     if (list == null) {
       list = new ArrayList<>();

playwright/src/test/java/com/microsoft/playwright/TestPageBasic.java

@@ -347,4 +347,13 @@ void shouldPropagateCloseReasonToPendingActions() {
     }));
     assertTrue(e.getMessage().contains("The reason."), e.getMessage());
   }
+
+  @Test
+  void shouldProhibitNullListeners() {
+    Page newPage = context.newPage();
+
+    PlaywrightException e = assertThrows(PlaywrightException.class, () -> newPage.onClose(null));
+
+    assertTrue(e.getMessage().contains("Can't add a null listener"));
+  }
 }