Can't use editorjs-columns

[necromind]

Can't use addon https://github.com/calumk/editorjs-columns

Saving failed due to the Error TypeError: this.editors.cols[t].save is not a function
    at I.save (editorjs-columns:8:469878)
    at T.<anonymous> (editorjs@2.25.0:8:42950)
    at l (editorjs@2.25.0:8:97783)
    at Generator._invoke (editorjs@2.25.0:8:97536)
    at forEach.t.<computed> [as next] (editorjs@2.25.0:8:98140)
    at n (editorjs@2.25.0:8:20378)
    at s (editorjs@2.25.0:8:20589)
    at editorjs@2.25.0:8:20648
    at new Promise (<anonymous>)
    at T.<anonymous> (editorjs@2.25.0:8:20529) %

tools={
    "Header": {
        'class': 'Header',
        'inlineToolbar': True,
        'config': {
            'placeholder': 'Enter a header',
            'levels': [2, 3, 4],
            'defaultLevel': 3,
        },
    },
    'columns': {
        'class': 'editorjsColumns',
        'config': {
            'tools': 'column_tools', # IMPORTANT! ref the column_tools
        }
    },
}

And in custom.js I have:

// first define the tools to be made avaliable in the columns
let column_tools = {
    header: Header,
    alert : Alert,
    paragraph : editorjsParagraphLinebreakable,
    delimiter : Delimiter
}

And plus button disappeared.
Can anyone help please?