"Help/Search" and "Help/Show Context Help" menu items do not work in Linux

## Introduction

We have the problem, as described in the title, in an application that uses these components. The same problem can be reproduced using Eclipse IDE. This problem occurs when using the windowing system **Wayland** but not X11.

I did some debugging, and it may be that `getActiveShell` returning `null` in [DefaultHelpUI.java](https://github.com/eclipse-platform/eclipse.platform/blob/07f86abab16a7b6014fd3adef3a3c9dae02da7ea/ua/org.eclipse.help.ui/src/org/eclipse/help/ui/internal/DefaultHelpUI.java#L198) is what is causing the problem:

```java
IWorkbenchWindow window = PlatformUI.getWorkbench().getActiveWorkbenchWindow();
Shell activeShell = getActiveShell();
if (window != null && isActiveShell(activeShell, window)) {
```

It worth noting that the problem occurs most of the time but not always.
## Steps to Reproduce

- Open Eclipse IDE
- Click on Help
- Click on "Search" or "Show Context Help"

I expected: help window to show up
But got: nothing

## Test Environment

- OS (as a virtual machine using VirtualBox)
- Debian GNU/Linux 12 (bookworm)
- Gnome 43.9
- Wayland
- Eclipse IDE
- Version: 2025-06 (4.36.0)
- Build ID: 20250605-1316