Added null checks for NPE on mSystemServerInterface

Bug: 119254124
Change-Id: Iebec84b414b2e156b9cdada05d66bfeb7f6caf05
Test: atest CtsContentCaptureServiceTestCases
diff --git a/core/java/android/view/contentcapture/MainContentCaptureSession.java b/core/java/android/view/contentcapture/MainContentCaptureSession.java
index baf4a35..d076af5 100644
--- a/core/java/android/view/contentcapture/MainContentCaptureSession.java
+++ b/core/java/android/view/contentcapture/MainContentCaptureSession.java
@@ -193,6 +193,8 @@
         final int flags = 0; // TODO(b/111276913): get proper flags
 
         try {
+            if (mSystemServerInterface == null) return;
+
             mSystemServerInterface.startSession(mContext.getUserId(), mApplicationToken,
                     componentName, mId, flags, new IResultReceiver.Stub() {
                         @Override
@@ -375,6 +377,8 @@
         }
 
         try {
+            if (mSystemServerInterface == null) return;
+
             mSystemServerInterface.finishSession(mContext.getUserId(), mId);
         } catch (RemoteException e) {
             Log.e(TAG, "Error destroying system-service session " + mId + " for "