Use random port for DevTools tests

Change the `RemoteApplicationLauncher` to use a random port and also
protect against an NPE if the launch fails.

spring-boot-tests/spring-boot-integration-tests/spring-boot-devtools-tests/src/test/java/org/springframework/boot/devtools/tests/DevToolsIntegrationTests.java

@@ -80,8 +80,10 @@ public class DevToolsIntegrationTests {
 
 	@After
 	public void stopApplication() throws InterruptedException {
+		if (this.launchedApplication != null) {
 			this.launchedApplication.stop();
 		}
+	}
 
 	@Test
 	public void addARequestMappingToAnExistingController() throws Exception {

spring-boot-tests/spring-boot-integration-tests/spring-boot-devtools-tests/src/test/java/org/springframework/boot/devtools/tests/RemoteApplicationLauncher.java

@@ -40,12 +40,12 @@ abstract class RemoteApplicationLauncher implements ApplicationLauncher {
 			throws Exception {
 		LaunchedJvm applicationJvm = javaLauncher.launch("app",
 				createApplicationClassPath(), "com.example.DevToolsTestApplication",
-				"--server.port=12345", "--spring.devtools.remote.secret=secret");
-		awaitServerPort(applicationJvm.getStandardOut());
+				"--server.port=0", "--spring.devtools.remote.secret=secret");
+		int port = awaitServerPort(applicationJvm.getStandardOut());
 		LaunchedJvm remoteSpringApplicationJvm = javaLauncher.launch(
 				"remote-spring-application", createRemoteSpringApplicationClassPath(),
 				RemoteSpringApplication.class.getName(),
-				"--spring.devtools.remote.secret=secret", "http://localhost:12345");
+				"--spring.devtools.remote.secret=secret", "http://localhost:" + port);
 		awaitRemoteSpringApplication(remoteSpringApplicationJvm.getStandardOut());
 		return new LaunchedApplication(new File("target/remote"),
 				applicationJvm.getStandardOut(), applicationJvm.getStandardError(),