Select TreeItem if none is selected

(cherry picked from commit 282639d6533a51ed395253ee72a9473b326f0b11)

scene/gui/tree.cpp

@@ -3191,6 +3191,9 @@ void Tree::item_selected(int p_column, TreeItem *p_item) {
 		//emit_signal("multi_selected",p_item,p_column,true); - NO this is for TreeItem::select
 
 		selected_col = p_column;
+		if (!selected_item) {
+			selected_item = p_item;
+		}
 	} else {
 
 		select_single_item(p_item, root, p_column);