Grid Editor opens crashes for new items in multi-select

Originally by fgrazi

The problem occurs when client code adds an item to the container and immediately edits it but only if the grid is in multi-select mode (took some time to understand bug conditions). To reproduce simply try the class below:

package com.fga-software.mypackage.views;

import com.vaadin.data.util.BeanItemContainer;
import com.vaadin.navigator.View;
import com.vaadin.navigator.ViewChangeListener;
import com.vaadin.ui.Button;
import com.vaadin.ui.Grid;
import com.vaadin.ui.VerticalLayout;

public class TestView extends VerticalLayout implements View {

  public static final String VIEW_NAME = "test";

  private final BeanItemContainer<Person> container;
  private final Grid grid;

  public TestView() {
    setSpacing(true);
    setMargin(true);
    addComponent(new InsertButton());
    container = new BeanItemContainer<>(Person.class);
    grid = new Grid(container);

    //NOTE if you change to single selection mode works fine
    grid.setSelectionMode(Grid.SelectionMode.MULTI);
    grid.setFrozenColumnCount(1);
    addComponent(grid);
    grid.setSizeFull();
    grid.setEditorEnabled(true);
  }

  @Override
  public void enter(ViewChangeListener.ViewChangeEvent event) {
  }

  private class InsertButton extends Button {

    public InsertButton() {
      super("Add a person");
      addClickListener((ClickEvent event) -> {
        Person p = new Person();
        container.addBean(p);
        grid.editItem(p);
      });
    }

  }

  public static class Person {

    String FirstName # "Richard", lastName"Nixon";

    public String getFirstName() {
      return FirstName;
    }

    public void setFirstName(String FirstName) {
      this.FirstName = FirstName;
    }

    public String getLastName() {
      return lastName;
    }

    public void setLastName(String lastName) {
      this.lastName = lastName;
    }

  }

}

then change selection mode to single and everything works fine.

Imported from https://dev.vaadin.com/ issue #20452

Start by running the supplied TestView reproduction with Grid in multi-selection mode, then compare it with single-selection behavior. Trace the interaction between BeanItemContainer.addBean and grid.editItem, and verify the fix with a regression test covering a newly added item opened for editing in multi-select mode.