Designer: Fix crash on close

...when executing plugin unit tests for Designer.

Change-Id: Ic3252bfef4559de4e54f5c3c3fd1984ab6a00f9d
Reviewed-by: Friedemann Kleint <Friedemann.Kleint@qt.io>
Reviewed-by: Jarek Kobus <jaroslaw.kobus@qt.io>

src/plugins/designer/formeditorstack.h

@@ -53,6 +53,7 @@ class FormEditorStack : public QStackedWidget
 
 public:
     explicit FormEditorStack(QWidget *parent = nullptr);
+    ~FormEditorStack() override;
 
     void add(const EditorData &d);