Fix NPE in MenuItem#calculateRenderSize for non-cascade items

Previously, calculateRenderSize created a GC using getMenu().getShell(),
which throws a NullPointerException when the MenuItem has no submenu
(e.g., PUSH, CHECK styles).

This change replaces getMenu().getShell() with getParent().getShell(),
ensuring a valid shell is always used regardless of item type.

fixes: #2247

Author: ShahzaibIbrahim

bundles/org.eclipse.swt/Eclipse SWT/win32/org/eclipse/swt/widgets/MenuItem.java

@@ -1425,7 +1425,7 @@ LRESULT wmMeasureChild (long wParam, long lParam) {
 }
 
 private Point calculateRenderedTextSize() {
-	GC gc = new GC(this.getMenu().getShell());
+	GC gc = new GC(this.getParent().getShell());
 	String textWithoutMnemonicCharacter = getText().replace("&", "");
 	Point points = gc.textExtent(textWithoutMnemonicCharacter);
 	gc.dispose();