Fix NPE when deserializing TestIdentifier (#3820)

An NPE was thrown when (de)serializing `TestIdentifiers` without a parent.

Fixes #3819.

Co-authored-by: Marc Philipp <mail@marcphilipp.de>

(cherry picked from commit bc55d0e)

documentation/src/docs/asciidoc/release-notes/release-notes-5.10.3.adoc

@@ -19,6 +19,9 @@ on GitHub.
   `@ConfigurationParameter` when selecting tests by `UniqueId`.
 * In order to support using `@EnabledInNativeImage` on test classes,
   `UniqueIdTrackingListener` now tracks descendants of skipped test containers.
+* Attempting to deserialize a `TestIdentifier` no longer causes a `NullPointerException`
+  when there is no parent identifier. See
+  link:https://github.com/junit-team/junit5/issues/3819[issue 3819].
 
 ==== Deprecations and Breaking Changes
 

junit-platform-launcher/src/main/java/org/junit/platform/launcher/TestIdentifier.java

@@ -284,7 +284,8 @@ private void readObject(ObjectInputStream s) throws ClassNotFoundException, IOEx
 		source = serializedForm.source;
 		tags = serializedForm.tags;
 		type = serializedForm.type;
-		parentId = UniqueId.parse(serializedForm.parentId);
+		String parentId = serializedForm.parentId;
+		this.parentId = parentId == null ? null : UniqueId.parse(parentId);
 		legacyReportingName = serializedForm.legacyReportingName;
 	}
 
@@ -307,7 +308,8 @@ private static class SerializedForm implements Serializable {
 
 		SerializedForm(TestIdentifier testIdentifier) {
 			this.uniqueId = testIdentifier.uniqueId.toString();
-			this.parentId = testIdentifier.parentId.toString();
+			UniqueId parentId = testIdentifier.parentId;
+			this.parentId = parentId == null ? null : parentId.toString();
 			this.displayName = testIdentifier.displayName;
 			this.legacyReportingName = testIdentifier.legacyReportingName;
 			this.source = testIdentifier.source;

platform-tests/src/test/java/org/junit/platform/launcher/TestIdentifierTests.java

@@ -72,6 +72,21 @@ void initialVersionCanBeDeserialized() throws Exception {
 		}
 	}
 
+	@Test
+	void identifierWithNoParentCanBeSerializedAndDeserialized() throws Exception {
+		TestIdentifier originalIdentifier = TestIdentifier.from(
+			new AbstractTestDescriptor(UniqueId.root("example", "id"), "Example") {
+				@Override
+				public Type getType() {
+					return Type.CONTAINER;
+				}
+			});
+
+		var deserializedIdentifier = (TestIdentifier) deserialize(serialize(originalIdentifier));
+
+		assertDeepEquals(originalIdentifier, deserializedIdentifier);
+	}
+
 	private static void assertDeepEquals(TestIdentifier first, TestIdentifier second) {
 		assertEquals(first, second);
 		assertEquals(first.getUniqueId(), second.getUniqueId());