Null Type Not Working With Enum

[aress31]

Prerequisites

• I have searched the existing issues
• I understand that providing a SSCCE example is tremendously useful to the maintainers.
• I have read the documentation
• Ideally, I'm providing a sample JSFiddle, Codesandbox.io or preferably a shared playground link demonstrating the issue.

What theme are you using?

mui

Version

5.x

Current Behavior

When defining a property with an enum, e.g.:

{
  "schema": {
    "type": "object",
    "definitions": {
      "status": {
        "enum": ["active", "cancelled", "completed", "inactive", "pending"]
      }
    },
    "properties": {
      // --- SNIP ---
      "status": {
        "type": ["string", "null"],
        "$ref": "#/definitions/status"
      },
      // --- SNIP ---
    }
  },
  "uiSchema": {
    // --- SNIP ---
  }
}

Null values are still not accepted, see:

Expected Behavior

No response

Steps To Reproduce

No response

Environment

Latest everything.

Anything else?

No response

[nickgros]

You can get null to appear as an option if you add null to the `"enum" property:

{
  "type": "object",
  "definitions": {
    "status": {
      "enum": [
        null,
        "active",
        "cancelled",
        "completed",
        "inactive",
        "pending"
      ]
    }
  },
  "properties": {
    "status": {
      "type": [
        "string",
        "null"
      ],
      "$ref": "#/definitions/status"
    }
  }
}

If you want to hide the null title, you can make null the default value for the property and either use enumNames in the uiSchema, or you should be able to reconstruct the enum using oneOf and const like in the below schema. However it looks like there is a bug where we don't use the title if it's an empty string, which we can track in #4448.

{
  "type": "object",
  "definitions": {
    "status": {
      "oneOf": [
        {
          "const": null,
          "title": ""
        },
        {
          "const": "active"
        },
        {
          "const": "cancelled"
        },
        {
          "const": "completed"
        },
        {
          "const": "inactive"
        },
        {
          "const": "pending"
        }
      ],
      "default": null
    }
  },
  "properties": {
    "status": {
      "type": [
        "string",
        "null"
      ],
      "$ref": "#/definitions/status"
    }
  }
}

We can discuss possible improvements & customizations to this functionality in your other issue #4421