问Android VpnService.Builder.establish()偶尔为空

EN

Attempt to invoke virtual method 'java.lang.String android.os.ParcelFileDescriptor.toString()' on a null object reference

// This is inside my VpnService implementation 

private ParcelFileDescriptor configureTunnelWithPushOpts(PushOpts popts)
{
    VpnService.Builder builder = this.new Builder();

    builder.setMtu       ( currentServerPrefs.SERVER_MTU );
    builder.addAddress   ( popts.ip, 32                  );
    builder.addDnsServer ( popts.dns1                    );
    builder.addDnsServer ( popts.dns2                    );
    builder.addRoute     ( "0.0.0.0", 0                  );


    // Note: Blocking mode is now enabled in native
    // code under the setFileDescriptor function.
    // builder.setBlocking(true);

    builder.setConfigureIntent(
            PendingIntent.getActivity(
                    this,
                    0,
                    new Intent(
                            this,
                            MainActivity.class
                    ),
            PendingIntent.FLAG_UPDATE_CURRENT)
    );

    final ParcelFileDescriptor vpnInterface;

    synchronized (this) {
        builder.setSession(currentServerPrefs.SERVER_ADDRESS);
        vpnInterface = builder.establish();
    }

    Logger.info("New interface: " + vpnInterface.toString(), this);
    return vpnInterface;
}

// This is inside my MainActivity class

Intent intent = VpnService.prepare(getApplicationContext());
if (intent != null) {
    startActivityForResult(intent, 0);
} else {
    onActivityResult(0, RESULT_OK, null);
}

. . . 

@Override
protected void onActivityResult(int requestCode, int resultCode, Intent data) {
    if (resultCode == RESULT_OK) {
        Intent intent = new Intent(this, MyVpnService.class);
        startService(intent);
    }
}