Handle null value returned by MtpDevice#getRoots.

    synchronized MtpRoot[] getRoots(int deviceId) throws IOException {

        final MtpDevice device = getDevice(deviceId);

        final int[] storageIds = device.getStorageIds();

        if (storageIds == null) {

            throw new IOException("Failed to obtain storage IDs.");

        }

        final MtpRoot[] results = new MtpRoot[storageIds.length];

        for (int i = 0; i < storageIds.length; i++) {

            results[i] = new MtpRoot(deviceId, device.getStorageInfo(storageIds[i]));