Fix NPE when deserializing TestIdentifier

Issue: #3819

Author: dmlloyd

documentation/src/docs/asciidoc/release-notes/release-notes-5.11.0-M2.adoc

@@ -26,6 +26,7 @@ repository on GitHub.
 [[release-notes-5.11.0-M2-junit-platform-bug-fixes]]
 ==== Bug Fixes
 
+* Fixed a bug where `TestIdentifier` could cause a `NullPointerException` on deserialize when there is no parent identifier. See link:https://github.com/junit-team/junit5/issues/3819[issue 3819].
 * ❓
 
 [[release-notes-5.11.0-M2-junit-platform-deprecations-and-breaking-changes]]

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

@@ -307,7 +307,8 @@ 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;