Handle multiple ContextRefreshedEvents in BackgroundPreinitializer

The same initializer will receive multiple ContextRefreshedEvents
when their is an application context hierarchy. This commit updates
the initializer to correctly handle multiple ContextRefreshedEvents
and only act upon the first that it receives.

See gh-4871

spring-boot-autoconfigure/src/main/java/org/springframework/boot/autoconfigure/BackgroundPreinitializer.java

@@ -53,7 +53,7 @@ public class BackgroundPreinitializer implements ApplicationListener<Application
 
 	private void performInitialization() {
 		try {
-			this.initializationThread = new Thread(new Runnable() {
+			Thread thread = new Thread(new Runnable() {
 
 				@Override
 				public void run() {
@@ -72,7 +72,8 @@ public class BackgroundPreinitializer implements ApplicationListener<Application
 				}
 
 			}, "background-preinit");
-			this.initializationThread.start();
+			thread.start();
+			this.initializationThread = thread;
 		}
 		catch (Exception ex) {
 			// This will fail on GAE where creating threads is prohibited. We can safely
@@ -82,14 +83,17 @@ public class BackgroundPreinitializer implements ApplicationListener<Application
 	}
 
 	private void awaitInitialization() {
-		try {
-			this.initializationThread.join();
-		}
-		catch (InterruptedException ex) {
-			Thread.currentThread().interrupt();
-		}
-		finally {
-			this.initializationThread = null;
+		Thread thread = this.initializationThread;
+		if (thread != null) {
+			try {
+				thread.join();
+			}
+			catch (InterruptedException ex) {
+				Thread.currentThread().interrupt();
+			}
+			finally {
+				this.initializationThread = null;
+			}
 		}
 	}