Use is_null() instead of !is_valid()

Co-authored-by: A Thousand Ships <[email protected]>

scene/main/viewport.cpp

@@ -1936,7 +1936,7 @@ void Viewport::_gui_input_event(Ref<InputEvent> p_event) {
 				// If the tooltip timer isn't running, start it.
 				// Otherwise, only reset the timer if the mouse has moved more than 5 pixels.
 				if (!is_tooltip_shown && over->can_process() &&
-						(!gui.tooltip_timer.is_valid() ||
+						(gui.tooltip_timer.is_null() ||
 								Math::is_zero_approx(gui.tooltip_timer->get_time_left()) ||
 								mm->get_relative().length() > 5.0)) {
 					if (gui.tooltip_timer.is_valid()) {