I have got something,

On file web/static/src/js/view_form.js, inside the class instance.web.form.FieldMany2ManyBinaryMultiFiles,

There is one method on_file_delete,

There is following code written,

    on_file_delete: function (event) {
        alert('testing deleting');
        event.stopPropagation();
        var file_id=$(event.target).data("id");
        if (file_id) {
            var files = _.filter(this.get('value'), function (id) {return id != file_id;});
            if(!this.data[file_id].no_unlink) {
                this.ds_file.unlink([file_id]);
            }
            this.set({'value': files});
        }
    },

If we remove this condition "if(!this.data[file_id].no_unlink)" then this error will solved.  

I am not sure why they have set no_unlink property.

What is usage of this ?

Anyone knows this ?

Any serious bug should be reported on GitHub, not on this board. Although it is very nice of you to share code on how to solve the bug.