Wt 3.2.2 dynamic widget adding/removing layout problem

#include <Wt/WApplication>
#include <Wt/WComboBox>
#include <Wt/WContainerWidget>
#include <Wt/WGridLayout>
#include <Wt/WVBoxLayout>
#include <Wt/WHBoxLayout>
#include <Wt/WGroupBox>
#include <Wt/WLabel>
#include <Wt/WText>
#include <Wt/WPushButton>
#include <Wt/WLineEdit>
#include <Wt/WPopupMenu>
#include <iostream>

class AttributeGroup;

class WtApp : public Wt::WApplication
{
   public:
      WtApp( const Wt::WEnvironment& env);
      ~WtApp() {}

   private:
      Wt::WText *createText(const Wt::WString& text);
      Wt::WWidget *createSelection();

      void choiceChanged(int i);

   private:
      Wt::WGridLayout* mLayout;
      Wt::WContainerWidget* mSelections;
      Wt::WVBoxLayout* mSelectionLayout;
      Wt::WText* mStretch;
      AttributeGroup* mAttrGroup;
};

class Attribute : public Wt::WGroupBox
{
   public:
      Attribute(const Wt::WString& title,
        const Wt::WString& description)
     : WGroupBox(title)
     , mDesc(description)
     , mLabel(new Wt::WLabel(mDesc))
     , mVBox(new Wt::WVBoxLayout())
     , mHBox(new Wt::WHBoxLayout())
      {
     mHBox->setContentsMargins(0, 0, 0, 0);
     mHBox->addWidget(mLabel, 0);
     mVBox->addLayout(mHBox);
     mVBox->setContentsMargins(0, 0, 0, 3);
     setLayout(mVBox);
     setStyleClass("attrbox");
     hide();
      }

      Attribute(const Wt::WString& title)
     : WGroupBox(title)
     , mDesc()
     , mLabel(0)
     , mVBox(new Wt::WVBoxLayout())
     , mHBox(0)
      {
     mVBox->setContentsMargins(0, 0, 0, 3);
     setLayout(mVBox);
     setStyleClass("attrbox");
     hide();
      }

   protected:
      Wt::WString mDesc;
      Wt::WLabel* mLabel;
      Wt::WVBoxLayout* mVBox;
      Wt::WHBoxLayout* mHBox;
};

class NameAttribute : public Attribute
{
   public:
      NameAttribute(const Wt::WString& name)
     : Attribute(Wt::WString("Edit ")+name,
             Wt::WString())
     , mName(new Wt::WLineEdit())
      {
     mHBox->addWidget(mName, 1);
     mLabel->setBuddy(mName);
      }

   private:
      Wt::WLineEdit* mName;
};

class ListAttribute : public Attribute
{
   public:
      ListAttribute(const Wt::WString& name,
            const std::vector<Wt::WString>& list)
     : Attribute(Wt::WString("Select ") + name,
             Wt::WString())
     , mList(new Wt::WComboBox())
      {
     for(size_t i=0; i<list.size(); ++i)
        mList->addItem(list[i]);
     mHBox->addWidget(mList, 1);
      }

   private:
      Wt::WComboBox* mList;
};

class AttributeGroup : public Wt::WGroupBox
{
   public:
      AttributeGroup()
     : WGroupBox(Wt::WString("Attribute Group"))
     , mVBox(new Wt::WVBoxLayout())
     , mAttrs()
     , mItemMap()
     , mResetPB(new Wt::WPushButton(Wt::WString("Reset")))
      {
     Wt::WPopupMenu *popup = new Wt::WPopupMenu();
     Wt::WLabel *menu = new Wt::WLabel(Wt::WString("Attributes"));
     menu->setStyleClass("label-popupmenu");
     menu->clicked().connect(popup, &Wt::WPopupMenu::popup);

     addPopupItem(popup, Wt::WString("Name"),
              new NameAttribute("Name"));
     addPopupItem(popup, Wt::WString("Street"),
              new NameAttribute(Wt::WString("Street")));
     std::vector<Wt::WString> continents;
     continents.push_back(Wt::WString("Europe"));
     continents.push_back(Wt::WString("Africa"));
     continents.push_back(Wt::WString("Asia"));
     continents.push_back(Wt::WString("America"));
     addPopupItem(popup, Wt::WString("Continents"),
              new ListAttribute(Wt::WString("Continent"),
                    continents));
     Wt::WPushButton *apply = new Wt::WPushButton(Wt::WString("Apply"));
     mResetPB->clicked().connect(this, &AttributeGroup::reset);
     mResetPB->disable();

     Wt::WHBoxLayout* hbox = new Wt::WHBoxLayout();
     hbox->setContentsMargins(0, 0, 0, 0);
     hbox->addWidget(menu, 1);
     hbox->addWidget(mResetPB, 1);
     hbox->addWidget(apply, 1);
     mVBox->addLayout(hbox);
     mVBox->setContentsMargins(0, 0, 0, 0);
     setLayout(mVBox);
     setStyleClass("selectionarea");
      }

      void addPopupItem(Wt::WPopupMenu* popup, const Wt::WString& text, Attribute* attr)
      {
     Wt::WPopupMenuItem* item = popup->addItem(text);
     mAttrs.push_back(attr);
     item->setCheckable(true);
     item->setChecked(false);
     item->setData(attr);
     item->triggered().connect(this, &AttributeGroup::attrChanged);
     mItemMap[attr] = item;
     mVBox->addWidget(attr);
      }

      void attrChanged(Wt::WPopupMenuItem* item)
      {
     Attribute *attr = reinterpret_cast<Attribute*>(item->data());
     attr->setHidden(!item->isChecked());
     mResetPB->setEnabled(true);
      }

      void reset()
      {
     for(std::list<Attribute*>::const_iterator it = mAttrs.begin(); it != mAttrs.end(); ++it)
     {
        Attribute* attr = (*it);
        attr->setHidden(true);
        mItemMap[attr]->setChecked(false);
     }
     mResetPB->setEnabled(false);
      }

   private:
      Wt::WVBoxLayout* mVBox;
      std::list<Attribute*> mAttrs;
      std::map<Attribute*,Wt::WPopupMenuItem*> mItemMap;
      Wt::WPushButton* mResetPB;
};


WtApp::WtApp(const Wt::WEnvironment& env)
   : Wt::WApplication(env)
   , mLayout(0)
{
   setCssTheme("polished");

   mLayout = new Wt::WGridLayout(root());

   root()->setLayout(mLayout);

   // row 0
   mLayout->addWidget(createText("Title"), 0, 0, 0, 3, Wt::AlignTop|Wt::AlignCenter);


   // row 1
   mLayout->addWidget(createText("SubTitle"), 1, 0, 1, 3, Wt::AlignMiddle|Wt::AlignCenter);

   // row 2
   mLayout->addWidget(createSelection(), 2, 0, Wt::AlignLeft);
   mLayout->addWidget(createText("Main Area"), 2, 1, Wt::AlignJustify);
   mLayout->addWidget(createText("Details"), 2, 2, Wt::AlignJustify);

   // row 3
   mLayout->addWidget(createText("More details"), 3, 0, 1, 3);

   /*
    * Let row 2 and column 1 take the excess space.
    */
   mLayout->setRowStretch(2, 1);
   mLayout->setRowStretch(3, 1);
   mLayout->setColumnStretch(1, 1);
   mLayout->setColumnStretch(2, 0);
   mLayout->setColumnResizable(1);
   mLayout->setRowResizable(2);
   mLayout->setHorizontalSpacing(4);
   mLayout->setVerticalSpacing(2);
   mLayout->setContentsMargins(4, 4, 4, 4);
}

Wt::WText *WtApp::createText(const Wt::WString& text)
{
   return new Wt::WText(text);
}

Wt::WWidget *WtApp::createSelection()
{
   mSelections = new Wt::WContainerWidget();
   Wt::WGroupBox *gbox1 = new Wt::WGroupBox("Config");
   gbox1->addWidget(createText("Config selection"));
   Wt::WComboBox *cbox1  = new Wt::WComboBox();
   cbox1->addItem("please select");
   cbox1->addItem("System");
   Wt::WGroupBox *gbox2 = new Wt::WGroupBox("Selection Box");
   Wt::WComboBox *cbox2  = new Wt::WComboBox();
   cbox2->addItem("please select");
   cbox2->addItem("Entity 1");
   cbox2->activated().connect(this, &WtApp::choiceChanged);
   mAttrGroup = new AttributeGroup();

   Wt::WVBoxLayout* gvbox2 = new Wt::WVBoxLayout();
   gvbox2->addWidget(createText("Test 1"), 0, Wt::AlignTop);
   gvbox2->addWidget(cbox2, 0, Wt::AlignTop);
   gbox2->setLayout(gvbox2);

   mStretch = createText("");
   mSelectionLayout = new Wt::WVBoxLayout();
   mSelectionLayout->setContentsMargins(4, 2, 4, 2);
   mSelectionLayout->addWidget(gbox1, 0, Wt::AlignTop);
   mSelectionLayout->addWidget(cbox1, 0, Wt::AlignTop);
   mSelectionLayout->addWidget(gbox2, 0, Wt::AlignTop);
   mSelectionLayout->addWidget(mAttrGroup, 0, Wt::AlignTop);
   mSelectionLayout->addWidget(mStretch, 1);
   mSelections->setContentAlignment(Wt::AlignJustify);
   mSelections->setLayout(mSelectionLayout);
   mSelections->setOverflow(Wt::WContainerWidget::OverflowAuto, Wt::Horizontal);
   mSelections->setStyleClass("inputarea");
   mSelections->setMinimumSize(Wt::WLength(150, Wt::WLength::Pixel), Wt::WLength());
   return mSelections;
}

void WtApp::choiceChanged(int i)
{
   mSelections->removeWidget(mStretch);
   mSelections->removeWidget(mAttrGroup);

   mSelectionLayout->addWidget(mAttrGroup, 0);
   mSelectionLayout->addWidget(mStretch, 1);
}

Wt::WApplication *createApplication(const Wt::WEnvironment& env)
{
   Wt::WApplication* myApplication = 0;
   myApplication = new WtApp( env );
   return myApplication;
}


int main(int argc, char **argv)
{
   Wt::WRun(argc, argv, &createApplication);
   return 0;
}