Add null check for PortNotifier

for onStartCommand error case or call `ACTION_STOP_VM_LAUNCHER_SERVICE`
before normal case.

Bug: n/a
Change-Id: I23803249df2e53835627ba562144d5a242abbce0
Test: build
diff --git a/android/TerminalApp/java/com/android/virtualization/terminal/VmLauncherService.java b/android/TerminalApp/java/com/android/virtualization/terminal/VmLauncherService.java
index a82c688..ee2a6b8 100644
--- a/android/TerminalApp/java/com/android/virtualization/terminal/VmLauncherService.java
+++ b/android/TerminalApp/java/com/android/virtualization/terminal/VmLauncherService.java
@@ -298,7 +298,9 @@
 
     @Override
     public void onDestroy() {
-        mPortNotifier.stop();
+        if (mPortNotifier != null) {
+            mPortNotifier.stop();
+        }
         getSystemService(NotificationManager.class).cancelAll();
         stopDebianServer();
         if (mVirtualMachine != null) {