NoSuchElement after external remove in background

When returning from Retouch:
```
java.util.NoSuchElementException
at space.taran.arklib.domain.meta.RootMetadataProcessor.retrieve-IoAF18A(RootMetadataProcessor.kt:41)
at space.taran.arknavigator.mvp.presenter.GalleryPresenter.fillGalleryItems(GalleryPresenter.kt:462)
at space.taran.arknavigator.domain.HandleGalleryExternalChangesUseCase$invoke$1$1.invokeSuspend(HandleGalleryExternalChangesUseCase.kt:29)
at kotlin.coroutines.jvm.internal.BaseContinuationImpl.resumeWith(ContinuationImpl.kt:33)
at kotlinx.coroutines.DispatchedTask.run(DispatchedTask.kt:106)
at android.os.Handler.handleCallback(Handler.java:938)
at android.os.Handler.dispatchMessage(Handler.java:99)
at android.os.Looper.loopOnce(Looper.java:201)
at android.os.Looper.loop(Looper.java:288)
at android.app.ActivityThread.main(ActivityThread.java:7870)
at java.lang.reflect.Method.invoke(Native Method)
at com.android.internal.os.RuntimeInit$MethodAndArgsCaller.run(RuntimeInit.java:548)
at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:1003)
Suppressed: kotlinx.coroutines.DiagnosticCoroutineContextException: [StandaloneCoroutine{Cancelling}@cee7c9b, Dispatchers.Main]
```

Start by reading RootMetadataProcessor.kt at line 41, then follow the call chain through GalleryPresenter.kt line 462 and HandleGalleryExternalChangesUseCase.kt line 29. Reproduce the crash by returning from Retouch after an external removal, and verify that gallery handling completes without a NoSuchElementException.

Written by the indexing model from the issue text.