Merge pull request #2068 from 821938089/fix-seekbar-bug

修复ReadBookActivity在一些第三方ROM上启动失败的bug

app/src/main/java/io/legado/app/ui/widget/seekbar/VerticalSeekBar.kt

@@ -69,7 +69,7 @@ class VerticalSeekBar @JvmOverloads constructor(context: Context, attrs: Attribu
         }
     }
 
-    override fun setThumb(thumb: Drawable) {
+    override fun setThumb(thumb: Drawable?) {
         mThumb = thumb
         super.setThumb(thumb)
     }