Fix NPE when viewHolder has been removed from RecyclerView

app/src/main/java/org/thoughtcrime/securesms/conversation/mutiselect/MultiselectItemAnimator.kt

@@ -131,12 +131,8 @@ class MultiselectItemAnimator(
   }
 
   override fun onAnimationFinished(viewHolder: RecyclerView.ViewHolder) {
-    dispatchItemDecorationRedraw(viewHolder)
-  }
-
-  private fun dispatchItemDecorationRedraw(viewHolder: RecyclerView.ViewHolder) {
-    val parent = (viewHolder.itemView.parent as RecyclerView)
-    parent.post { parent.invalidateItemDecorations() }
+    val parent = (viewHolder.itemView.parent as? RecyclerView)
+    parent?.post { parent.invalidateItemDecorations() }
   }
 
   private fun endSelectedAnimation(item: RecyclerView.ViewHolder) {