Error: Call to a member function getUID() on null

I have one machine nextcloud 11.0.2 and one collabora 2.0.4. Everything seems like working fine except that when someone shared document file located at local external storage, then other users got error messages when tried to clicked shared link to open file, the error message is below. I see in the log file, it said two issues like this:

Internal Server Error

I traced Request ID: jvFXjAdZI88dUDRja2MQ on nextcloud.log and saw that:
{“reqId”:“jvFXjAdZI88dUDRja2MQ”,“remoteAddr”:“10.36.60.222”,“app”:“index”,“message”:“Exception: {"Exception":"Error","Message":"Call to a member function getUID() on null","Code":0,"Trace":"#0 \/var\/www\/nextcloud\/apps\/richdocuments\/lib\/Controller\/DocumentController.php(153): OCA\Richdocuments\TokenManager->getToken(*** sensitive parameters replaced ***)\n#1 [internal function]: OCA\Richdocuments\Controller\DocumentController->publicPage(‘LpmRKObFuPZC6Wj’, ‘NextCloud_Endus…’)\n#2 \/var\/www\/nextcloud\/lib\/private\/AppFramework\/Http\/Dispatcher.php(160): call_user_func_array(Array, Array)\n#3 \/var\/www\/nextcloud\/lib\/private\/AppFramework\/Http\/Dispatcher.php(90): OC\AppFramework\Http\Dispatcher->executeController(Object(OCA\Richdocuments\Controller\DocumentController), ‘publicPage’)\n#4 \/var\/www\/nextcloud\/lib\/private\/AppFramework\/App.php(114): OC\AppFramework\Http\Dispatcher->dispatch(Object(OCA\Richdocuments\Controller\DocumentController), ‘publicPage’)\n#5 \/var\/www\/nextcloud\/lib\/private\/AppFramework\/Routing\/RouteActionHandler.php(47): OC\AppFramework\App::main(‘OCA\\Richdocumen…’, ‘publicPage’, Object(OC\AppFramework\DependencyInjection\DIContainer), Array)\n#6 [internal function]: OC\AppFramework\Routing\RouteActionHandler->_invoke(Array)\n#7 \/var\/www\/nextcloud\/lib\/private\/Route\/Router.php(299): calluser_func(Object(OC\AppFramework\Routing\RouteActionHandler), Array)\n#8 \/var\/www\/nextcloud\/lib\/base.php(1010): OC\Route\Router->match(‘\/apps\/richdocum…’)\n#9 \/var\/www\/nextcloud\/index.php(40): OC::handleRequest()\n#10 {main}","File":"\/var\/www\/nextcloud\/apps\/richdocuments\/lib\/TokenManager.php","Line":93}”,“level”:3,“time”:“2017-04-28T10:14:09+00:00”,“method”:“GET”,“url”:“/apps/richdocuments/public?shareToken=LpmRKObFuPZC6Wj&fileName=NextCloud_Enduser_Guide.docx&requesttoken=Vm80YkH2qnsyyKXqm5O49gbyTw2txHJL%2Bn218TILh5I%3D%3AIh5BKAC62R14nvae6eb33XbCIDX%2BikUpqBf2gEJo5fQ%3D”,“user”:“–”,“version”:“11.0.2.7”}

Error: Call to a member function getUID() on null`Prefo

Please help me to fix. Thanks in advance

Best Regards,
Bin