Merge pull request #10941 from izeye:null-check

* pr/10941:
  Add missing null check in FileSystemWatcher.stopAfter()

spring-boot-devtools/src/main/java/org/springframework/boot/devtools/filewatch/FileSystemWatcher.java

@@ -194,7 +194,7 @@ public class FileSystemWatcher {
 			}
 			this.watchThread = null;
 		}
-		if (Thread.currentThread() != thread) {
+		if (thread != null && Thread.currentThread() != thread) {
 			try {
 				thread.join();
 			}