Fix tooltips behaving incorrectly on Tree nodes

scene/main/viewport.h

@@ -368,6 +368,7 @@ private:
 		Control *tooltip_control = nullptr;
 		Window *tooltip_popup = nullptr;
 		Label *tooltip_label = nullptr;
+		String tooltip_text;
 		Point2 tooltip_pos;
 		Point2 last_mouse_pos;
 		Point2 drag_accum;