WorkingSetActionProvider: fix "Widget is disposed"

as logged during LabelProviderTest

asyncExec can happen when viewer is already disposed

Author: jukzi

bundles/org.eclipse.ui.navigator.resources/src/org/eclipse/ui/internal/navigator/resources/actions/WorkingSetActionProvider.java

@@ -329,6 +329,9 @@ public void restoreState(final IMemento aMemento) {
 
 		// Need to run this async to avoid being reentered when processing a selection change
 		viewer.getControl().getShell().getDisplay().asyncExec(() -> {
+			if (viewer.getControl().isDisposed()) {
+				return;
+			}
 			boolean showWorkingSets = true;
 			if (aMemento != null) {
 				Integer showWorkingSetsInt = aMemento