Fix fast scroller NPE

app/src/main/java/org/koitharu/kotatsu/base/ui/list/fastscroll/FastScroller.kt

@@ -516,6 +516,6 @@ class FastScroller @JvmOverloads constructor(
 
 	interface SectionIndexer {
 
-		fun getSectionText(context: Context, position: Int): CharSequence
+		fun getSectionText(context: Context, position: Int): CharSequence?
 	}
 }

app/src/main/java/org/koitharu/kotatsu/details/ui/adapter/ChaptersAdapter.kt

@@ -42,7 +42,8 @@ class ChaptersAdapter(
 		}
 	}
 
-	override fun getSectionText(context: Context, position: Int): CharSequence {
+	override fun getSectionText(context: Context, position: Int): CharSequence? {
-		return items[position].chapter.number.toString()
+		val item = items.getOrNull(position) ?: return null
+		return item.chapter.number.toString()
 	}
 }

app/src/main/java/org/koitharu/kotatsu/history/ui/HistoryListAdapter.kt

@@ -14,14 +14,14 @@ class HistoryListAdapter(
 	listener: MangaListListener
 ) : MangaListAdapter(coil, lifecycleOwner, listener), FastScroller.SectionIndexer {
 
-	override fun getSectionText(context: Context, position: Int): CharSequence {
+	override fun getSectionText(context: Context, position: Int): CharSequence? {
 		val list = items
 		for (i in (0..position).reversed()) {
-			val item = list[i]
+			val item = list.getOrNull(i) ?: continue
 			if (item is DateTimeAgo) {
 				return item.format(context.resources)
 			}
 		}
-		return ""
+		return null
 	}
 }

app/src/main/java/org/koitharu/kotatsu/shelf/ui/adapter/ShelfAdapter.kt

@@ -46,9 +46,9 @@ class ShelfAdapter(
 			.addDelegate(errorStateListAD(listener))
 	}
 
-	override fun getSectionText(context: Context, position: Int): CharSequence {
+	override fun getSectionText(context: Context, position: Int): CharSequence? {
-		val item = items.getOrNull(position) as? ShelfSectionModel
+		val item = items.getOrNull(position) as? ShelfSectionModel ?: return null
-		return item?.getTitle(context.resources) ?: ""
+		return item.getTitle(context.resources)
 	}
 
 	private class DiffCallback : DiffUtil.ItemCallback<ListModel>() {