Fix NPE

NPE happens when there is an orphaned session which we've
tried to prevent in all cases.

Log an error message if this situation happens.

Bug: 227342978
Test: atest CtsRootPackageInstallerHostTestCases
Change-Id: Ia21323926bd9db1a6f05461904deb45b4c3dd0bc
(cherry picked from commit 07e31dfb1e)
Merged-In: Ia21323926bd9db1a6f05461904deb45b4c3dd0bc

services/core/java/com/android/server/pm/PackageInstallerService.java

@@ -332,7 +332,10 @@ public class PackageInstallerService extends IPackageInstaller.Stub implements
                     // Their staging dirs will be removed too
                     PackageInstallerSession root = !session.hasParentSessionId()
                             ? session : mSessions.get(session.getParentSessionId());
-                    if (!root.isDestroyed() && 
+                    if (root == null) {
+                        Slog.e(TAG, "freeStageDirs: found an orphaned session: "
+                                + session.sessionId + " parent=" + session.getParentSessionId());
+                    } else if (!root.isDestroyed() && 
                             (!root.isStaged() || (root.isStaged() && root.isStagedSessionReady()))) 
                     {
                         root.abandon();