Avoid duplicated messages when activity is in background.

onDestroy is not called when navigating to the next activity or bringing the app to background. That's why the adapter was not set to null and all messages were added another time the next time the activity comes to foreground. With this fix, the adapter is set to null in onStop.

All in all, the adapter handling may not be the best, but will be replaced by Compose anyway..

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

app/src/main/java/com/nextcloud/talk/chat/ChatActivity.kt

@@ -562,6 +562,7 @@ class ChatActivity :
     override fun onStop() {
         super.onStop()
         active = false
+        adapter = null
         this.lifecycle.removeObserver(AudioUtils)
         this.lifecycle.removeObserver(chatViewModel)
     }
@@ -2677,7 +2678,6 @@ class ChatActivity :
 
         currentlyPlayedVoiceMessage?.let { stopMediaPlayer(it) } // FIXME, mediaplayer can sometimes be null here
 
-        adapter = null
         disposables.dispose()
     }