Find/replace overlay: avoid attempt to set focus while disposing overlay

When closing a find/replace overlay, the SWT dispose operations try to
set focus to some remaining widget. The HistoryTextWrapper tries to pass
this operation to the contained Text widget without validating it for
already being disposed (which is the case when closing the overlay),
thus potentially leading to exception.

This change ensures that before trying to set focus on the Text widget
of a HistoryTextWrapper that widget is validated for not being disposed.

bundles/org.eclipse.ui.workbench.texteditor/src/org/eclipse/ui/internal/findandreplace/overlay/HistoryTextWrapper.java

@@ -192,7 +192,10 @@ public void setForeground(Color color) {
 
 	@Override
 	public boolean forceFocus() {
-		return textBar.forceFocus();
+		if (!textBar.isDisposed()) {
+			return textBar.forceFocus();
+		}
+		return false;
 	}
 
 	@Override