[GTK4] Fix GtkWidget warning when updating shell bounds

This is a continuation of bd1cdd6 where
a 'GTK_IS_WIDGET (widget)' warning is logged when measuring the height
of a NULL titlebar. This issue appears when e.g. calling setBounds() on
a Shell.

To reproduce this warning, run e.g. Snippet16.

Author: ptziegler

bundles/org.eclipse.swt/Eclipse SWT/gtk/org/eclipse/swt/widgets/Shell.java

@@ -1856,9 +1856,11 @@ long gtk_size_allocate (long widget, long allocation) {
 			long display = GDK.gdk_display_get_default();
 			long monitor = GDK.gdk_display_get_monitor_at_surface(display, paintSurface());
 			GDK.gdk_monitor_get_geometry(monitor, monitorSize);
-			long header = GTK4.gtk_widget_get_next_sibling(GTK4.gtk_widget_get_first_child(shellHandle));
+			long header = GTK4.gtk_window_get_titlebar(shellHandle);
 			int[] headerNaturalHeight = new int[1];
-			GTK4.gtk_widget_measure(header, GTK.GTK_ORIENTATION_VERTICAL, -1, null, headerNaturalHeight, null, null);
+			if (header != 0) {
+				GTK4.gtk_widget_measure(header, GTK.GTK_ORIENTATION_VERTICAL, 0, null, headerNaturalHeight, null, null);
+			}
 			widthA[0] = monitorSize.width;
 			heightA[0] = monitorSize.height - headerNaturalHeight[0];
 		}
@@ -2371,9 +2373,11 @@ int setBounds (int x, int y, int width, int height, boolean move, boolean resize
 				 * On GTK4, GtkWindow size includes the header bar. In order to keep window size allocation of the client area
 				 * consistent with previous versions of SWT, we need to include the header bar height in addition to the given height value.
 				 */
-				long header = GTK4.gtk_widget_get_next_sibling(GTK4.gtk_widget_get_first_child(shellHandle));
+				long header = GTK4.gtk_window_get_titlebar(shellHandle);
 				int[] headerNaturalHeight = new int[1];
-				GTK4.gtk_widget_measure(header, GTK.GTK_ORIENTATION_VERTICAL, -1, null, headerNaturalHeight, null, null);
+				if (header != 0) {
+					GTK4.gtk_widget_measure(header, GTK.GTK_ORIENTATION_VERTICAL, 0, null, headerNaturalHeight, null, null);
+				}
 				GTK.gtk_window_set_default_size(shellHandle, width, height + headerNaturalHeight[0]);
 			} else {
 				GTK3.gtk_window_resize (shellHandle, width, height);