fix(coap-core): handle null in getCustomOption method

Author: szysas

coap-core/src/main/java/com/mbed/coap/packet/BasicHeaderOptions.java

@@ -1,5 +1,5 @@
 /*
- * Copyright (C) 2022-2025 java-coap contributors (https://github.com/open-coap/java-coap)
+ * Copyright (C) 2022-2026 java-coap contributors (https://github.com/open-coap/java-coap)
  * Copyright (C) 2011-2021 ARM Limited. All rights reserved.
  * SPDX-License-Identifier: Apache-2.0
  * Licensed under the Apache License, Version 2.0 (the "License");
@@ -133,7 +133,7 @@ protected boolean parseOption(int type, Opaque data) {
      * @return byte array value or null if does not exist
      */
     public Opaque getCustomOption(Integer optNumber) {
-        if (!unrecognizedOptions.containsKey(optNumber)) {
+        if (unrecognizedOptions == null || !unrecognizedOptions.containsKey(optNumber)) {
             return null;
         }
         return unrecognizedOptions.get(optNumber).getFirstValue();

coap-core/src/test/java/com/mbed/coap/packet/HeaderOptionsTest.java

@@ -1,5 +1,5 @@
 /*
- * Copyright (C) 2022-2025 java-coap contributors (https://github.com/open-coap/java-coap)
+ * Copyright (C) 2022-2026 java-coap contributors (https://github.com/open-coap/java-coap)
  * Copyright (C) 2011-2021 ARM Limited. All rights reserved.
  * SPDX-License-Identifier: Apache-2.0
  * Licensed under the Apache License, Version 2.0 (the "License");
@@ -533,6 +533,13 @@ void correlationOptionShouldBeElectiveAndSafeToForward() {
         assertFalse(hasNoCacheKey(HeaderOptions.OPEN_COAP_CORRELATION_TAG));
     }
 
+    @Test
+    void getCustomOption_shouldReturnNull_whenNoCustomOptionsSet() {
+        BasicHeaderOptions hdr = new BasicHeaderOptions();
+
+        assertNull(hdr.getCustomOption(100));
+    }
+
     private static byte[] serialize(BasicHeaderOptions hdr) throws IOException, CoapException {
         ByteArrayOutputStream baos = new ByteArrayOutputStream();
         hdr.serialize(baos);