Fix crash on random button click with empty list

app/src/main/kotlin/com/hippo/ehviewer/dao/LocalFavoritesDao.kt

@@ -27,7 +27,7 @@ interface LocalFavoritesDao {
     fun joinListLazy(title: String): PagingSource<Int, BaseGalleryInfo>
 
     @Query("SELECT GALLERIES.* FROM LOCAL_FAVORITES JOIN GALLERIES USING(GID) ORDER BY RANDOM() LIMIT 1")
-    suspend fun random(): BaseGalleryInfo
+    suspend fun random(): BaseGalleryInfo?
 
     @Query("SELECT EXISTS(SELECT * FROM LOCAL_FAVORITES WHERE GID = :gid)")
     suspend fun contains(gid: Long): Boolean

app/src/main/kotlin/com/hippo/ehviewer/ui/screen/DownloadsScreen.kt

@@ -648,7 +648,9 @@ fun AnimatedVisibilityScope.DownloadsScreen(navigator: DestinationsNavigator) =
     ) {
         if (!selectMode) {
             onClick(Icons.Default.Shuffle) {
-                withUIContext { navToReader(list.random().galleryInfo) }
+                if (list.isNotEmpty()) {
+                    withUIContext { navToReader(list.random().galleryInfo) }
+                }
             }
             onClick(Icons.AutoMirrored.Default.Sort) {
                 val oldMode = SortMode.from(sortMode)

app/src/main/kotlin/com/hippo/ehviewer/ui/screen/FavoritesScreen.kt

@@ -356,8 +356,9 @@ fun AnimatedVisibilityScope.FavouritesScreen(navigator: DestinationsNavigator) =
         if (!selectMode) {
             if (isLocalFav) {
                 onClick(Icons.Default.Shuffle) {
-                    val random = EhDB.randomLocalFav()
-                    withUIContext { navigate(random.asDst()) }
+                    EhDB.randomLocalFav()?.let { info ->
+                        withUIContext { navigate(info.asDst()) }
+                    }
                 }
             }
             onClick(EhIcons.Default.GoTo) {