Fix NPE when unregistering a backup transport.

        }

    }

    // Add a transport to our set of available backends

    // Add a transport to our set of available backends.  If 'transport' is null, this

    // is an unregistration, and the transport's entry is removed from our bookkeeping.

    private void registerTransport(String name, IBackupTransport transport) {

        synchronized (mTransports) {

            if (DEBUG) Log.v(TAG, "Registering transport " + name + " = " + transport);

            if (transport != null) {

                mTransports.put(name, transport);

            } else {

                mTransports.remove(name);

                if (mCurrentTransport != null && mCurrentTransport.equals(name)) {

                    mCurrentTransport = null;

                }

                // Nothing further to do in the unregistration case

                return;

            }

        }

        // If the init sentinel file exists, we need to be sure to perform the init