Cover Update Followup to Address #3139 (#3150)

* update cover logic when thumbnail url becomes null

* always clear cover on refresh even if custom cover is set

* remove concurrency changes

app/src/main/java/eu/kanade/tachiyomi/ui/library/LibraryController.kt

@@ -34,6 +34,7 @@ import eu.kanade.tachiyomi.ui.base.controller.withFadeTransaction
 import eu.kanade.tachiyomi.ui.main.MainActivity
 import eu.kanade.tachiyomi.ui.main.offsetAppbarHeight
 import eu.kanade.tachiyomi.ui.manga.MangaController
+import eu.kanade.tachiyomi.util.hasCustomCover
 import eu.kanade.tachiyomi.util.system.getResourceColor
 import eu.kanade.tachiyomi.util.system.toast
 import eu.kanade.tachiyomi.util.view.visible
@@ -487,7 +488,7 @@ class LibraryController(
     private fun handleChangeCover() {
         val manga = selectedMangas.firstOrNull() ?: return
 
-        if (coverCache.getCustomCoverFile(manga).exists()) {
+        if (manga.hasCustomCover(coverCache)) {
             showEditCoverDialog(manga)
         } else {
             openMangaCoverPicker(manga)