avoid ClassCastException on ItemLongClick in ConversationsListActivity

could happen when using long click on header when using the conversation search

Exception java.lang.ClassCastException:
  at com.nextcloud.talk.conversationlist.ConversationsListActivity.onItemLongClick (ConversationsListActivity.kt:1027)
  at eu.davidea.viewholders.FlexibleViewHolder.onLongClick (FlexibleViewHolder.java:144)
  at android.view.View.performLongClickInternal (View.java:8240)
  at android.view.View.performLongClick (View.java:8198)
  at android.view.View.performLongClick (View.java:8216)
  at android.view.View$CheckForLongPress.run (View.java:30194)
  at android.os.Handler.handleCallback (Handler.java:938)
  at android.os.Handler.dispatchMessage (Handler.java:99)
  at android.os.Looper.loop (Looper.java:246)
  at android.app.ActivityThread.main (ActivityThread.java:8653)
  at java.lang.reflect.Method.invoke
  at com.android.internal.os.RuntimeInit$MethodAndArgsCaller.run (RuntimeInit.java:602)
  at com.android.internal.os.ZygoteInit.main (ZygoteInit.java:1130)

Signed-off-by: Marcel Hibbe <dev@mhibbe.de>

app/src/main/java/com/nextcloud/talk/conversationlist/ConversationsListActivity.kt

@@ -1067,8 +1067,8 @@ class ConversationsListActivity :
             Log.d(TAG, "sharing to multiple rooms not yet implemented. onItemLongClick is ignored.")
         } else {
             val clickedItem: Any? = adapter!!.getItem(position)
-            if (clickedItem != null) {
-                val conversation = (clickedItem as ConversationItem).model
+            if (clickedItem != null && clickedItem is ConversationItem) {
+                val conversation = clickedItem.model
                 conversationsListBottomDialog = ConversationsListBottomDialog(
                     this,
                     userManager.currentUser.blockingGet(),