[BUG] Play console crash: NPE refreshFolderUseCase

Not many appearances, not critical. This is the stacktrace:

Exception java.lang.NullPointerException:
  at com.owncloud.android.ui.ReceiveExternalFilesViewModel.refreshFolderUseCase (Unknown Source:6)
  at com.owncloud.android.ui.activity.ReceiveExternalFilesActivity.startSyncFolderOperation (ReceiveExternalFilesActivity.java:627)
  at com.owncloud.android.ui.activity.ReceiveExternalFilesActivity.onBackPressed (ReceiveExternalFilesActivity.java:462)
  at com.owncloud.android.ui.activity.ReceiveExternalFilesActivity.onOptionsItemSelected (ReceiveExternalFilesActivity.java:784)
  at android.app.Activity.onMenuItemSelected (Activity.java:4834)
  at androidx.activity.ComponentActivity.onMenuItemSelected (ComponentActivity.java:529)
  at androidx.fragment.app.FragmentActivity.onMenuItemSelected (FragmentActivity.java:265)
  at androidx.appcompat.app.AppCompatActivity.onMenuItemSelected (AppCompatActivity.java:269)
  at androidx.appcompat.view.WindowCallbackWrapper.onMenuItemSelected (WindowCallbackWrapper.java:110)
  at androidx.appcompat.widget.ToolbarWidgetWrapper$1.onClick (ToolbarWidgetWrapper.java:188)
  at android.view.View.performClick (View.java:8230)
  at android.view.View.performClickInternal (View.java:8203)
  at android.view.View.-$$Nest$mperformClickInternal (Unknown Source)
  at android.view.View$PerformClick.run (View.java:32866)
  at android.os.Handler.handleCallback (Handler.java:959)
  at android.os.Handler.dispatchMessage (Handler.java:100)
  at android.os.Looper.loopOnce (Looper.java:249)
  at android.os.Looper.loop (Looper.java:337)
  at android.app.ActivityThread.main (ActivityThread.java:9497)
  at java.lang.reflect.Method.invoke
  at com.android.internal.os.RuntimeInit$MethodAndArgsCaller.run (RuntimeInit.java:636)
  at com.android.internal.os.ZygoteInit.main (ZygoteInit.java:1005)