NullPointerException when "delete a empty word"

[ssssy1000]

Environment

Device: pixel (Android Emulator)
Android Version: Android6.0
App Version: 10.11.7

Reproduction

1. Go to "Open"
2. Add any db
3. Click the new db
4. Preview/edit the db
5. click "delete" on the empty word page

ScreenShots

Exception log

05-12 02:19:29.256 25382-25382/org.liberty.android.fantastischmemo E/AndroidRuntime: FATAL EXCEPTION: main
    Process: org.liberty.android.fantastischmemo, PID: 25382
    **java.lang.NullPointerException**: Attempt to invoke virtual method 'java.lang.Class java.lang.Object.getClass()' on a null object reference
          at com.google.common.base.Preconditions.checkNotNull(Preconditions.java:1)
          at org.liberty.android.fantastischmemo.ui.PreviewEditActivity.deleteCard(PreviewEditActivity.java:1)
          at org.liberty.android.fantastischmemo.ui.PreviewEditActivity.onOptionsItemSelected(PreviewEditActivity.java:64)
          at android.app.Activity.onMenuItemSelected(Activity.java:2908)
          at androidx.fragment.app.FragmentActivity.onMenuItemSelected(FragmentActivity.java:1)
          at androidx.appcompat.app.AppCompatActivity.onMenuItemSelected(AppCompatActivity.java:1)
          at androidx.appcompat.view.WindowCallbackWrapper.onMenuItemSelected(WindowCallbackWrapper.java:1)
          at androidx.appcompat.app.AppCompatDelegateImpl.onMenuItemSelected(AppCompatDelegateImpl.java:4)
          at androidx.appcompat.view.menu.MenuBuilder.dispatchMenuItemSelected(MenuBuilder.java:1)
          at androidx.appcompat.view.menu.MenuItemImpl.invoke(MenuItemImpl.java:2)
          at androidx.appcompat.view.menu.MenuBuilder.performItemAction(MenuBuilder.java:4)
          at androidx.appcompat.view.menu.MenuBuilder.performItemAction(MenuBuilder.java:1)
          at androidx.appcompat.widget.ActionMenuView.invokeItem(ActionMenuView.java:1)
          at androidx.appcompat.view.menu.ActionMenuItemView.onClick(ActionMenuItemView.java:2)
          at android.view.View.performClick(View.java:5198)
          at android.view.View$PerformClick.run(View.java:21147)
          at android.os.Handler.handleCallback(Handler.java:739)
          at android.os.Handler.dispatchMessage(Handler.java:95)
          at android.os.Looper.loop(Looper.java:148)
          at android.app.ActivityThread.main(ActivityThread.java:5417)
          at java.lang.reflect.Method.invoke(Native Method)
          at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:726)
          at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:616)