Check action mode is still valid.

Due to support library bug, onActionItemClicked can be called after finish/onDestroyActionMode so we can check for that by checking if our reference to the action mode is still valid.

Fixes #8891

src/org/thoughtcrime/securesms/conversation/ConversationFragment.java

@@ -1054,6 +1054,8 @@ public class ConversationFragment extends Fragment
 
     @Override
     public boolean onActionItemClicked(ActionMode mode, MenuItem item) {
+      if (actionMode == null) return false;
+
       switch(item.getItemId()) {
         case R.id.menu_context_copy:
           handleCopyMessage(getListAdapter().getSelectedItems());