Unselect selected forum post when changing forums

briar-desktop/src/main/kotlin/org/briarproject/briar/desktop/forums/ThreadedConversationViewModel.kt

@@ -81,6 +81,7 @@ class ThreadedConversationViewModel @Inject constructor(
     @UiExecutor
     fun setGroupItem(groupItem: GroupItem) {
         this.groupItem = groupItem
+        _selectedPost.value = null
         loadPosts(groupItem.id)
     }