GetFolderAsync applied to the FutureAccessList delivers error message 2148734499

StorageItemAccessList _lAccess = StorageApplicationPermissions.FutureAccessList;

ApplicationDataContainer _localSettings = ApplicationData.Current.LocalSettings;

StorageFolder _Public; // _Public captured by the FolderPicker

StorageFolder _Private; // _Private captured by the FolderPicker

_localSettings.Values [ "Public" ] = _lAccess.Add ( _Public );

_localSettings.Values [ "Private" ] = _lAccessd.Add ( _Private );

private async Task ConnectWorkspace ()

{

 _localSettings.Values [ "workspace" ] = null;

  try

  {

        _Public = await _lAccess.GetFolderAsync ( ( string ) _localSettings.Values [ "Public" ] );

        await SetPublicWorkspace ();         // sets the public usere environment 

                                                               // sets _localSettings.Values [ "workspace" ] = "public" 

  }

  catch ( Exception ex )

  {

        _Public = null;

  }

  try

  {

        _Private = await _lAccess.GetFoldxerAsync ( ( string ) _localSettings.Values [ "Private ] );

        if ( ( string ) _localSettings.Values [ "workspace" ] == "public" )

              return;

        await SetPrivateWorkSpace ();        // sets the private user environment

                                                                // sets _localSettings.Values [ "workspace" ] = "private"

  }

  catch ( Exception  ex )

  {

        _Private = null;

  }

}

1. What is the purpose: my UWP uses two workspaces, a public and a private one. when a user working in the public workspace and moves outside of the WLAN-reach the UWP shall be capable to detect this and change automatically to the private workspace. In case the user returns within the WLAN-reach the UWP shall automatically change to the the public workspace.
2. What I expected: the above code snippet directs the UWP into the appropriate workspace without any interuption of it's normal operation. The realize this, the catch-branch has to be reached in all cases a worspace folder is not reacheable ( because it has been deleted or shifted to another PC )
3. What happens: when I e.g. delete the public workspace folder _Public or I shift it to another PC the UWP crashes with error 2148734499 at _LAccess.GetFolderAsync ( .. ) and does'nt reach the catch-branch
4. What I did: I made a small UWP with the following functions:

• capture a folder at the first start using FolderPicker and store it in the FutgureAccessList
• at all subsequent starts try to recover the folder from the FutureAccessyList and report errors
• the result was that await_lAccess.GetFolderAsync ( .. ) always delivered the expected result, it either showed me the folder's path or ( reaching the catch-branch ) reported an error I'm helpless, I can't explain the difference in the behaviors.

          I.m looking forward to Your help and thank You in advance