CheckboxesWidget should ensure that the value is an Array

[ggrossetie]

Prerequisites

• I have read the documentation;
• In the case of a bug report, I understand that providing a SSCCE example is tremendously useful to the maintainers.
• Ideally, I'm providing a sample JSFiddle or a shared playground link demonstrating the issue.

Description

CheckboxesWidget assumes that the value is an Array but does not check the type of the value.
At runtime, the widget will throw an exception when a value is selected or deselected.

Steps to Reproduce

See the codesandbox example: https://codesandbox.io/s/checkboxeswidget-with-non-array-value-react-jsonschema-form-qi96b?file=/src/App.js

Important parts:

var formData = {
  typeArticle: "Essai"
};

Expected behavior

The widget should check that the value is an Array. If not, the widget could either create an Array from the value or ignore the value.

Actual behavior

Throws an exception when the user is trying to select or deselect a value.
Please note that the value passed as a String will be correctly selected.

In the above example, the value "Essai" is selected (even if the value was Essai and not ["Essai"]):

Version

2.4.0