Online demo does not handle DOCTYPE correctly and gives an error

[Edgar-P-yan]

• [x] Are you running the latest version?
• [x] Have you included sample input, output, error, and expected output?
• [x] Have you checked if you are using correct configuration?
• [x] Did you try online tool?

Description

Steps to reproduce:

1. go to the online tool https://naturalintelligence.github.io/fast-xml-parser/
2. no additional input needed, just click "Parse to JSON"
3. then "Parse to XML"
4. and again "Parse to JSON"
5. it'll print Error: XML declaration allowed only at the start of the document.:2:6

I guess this is due to the { "?xml": "", ... } property, which converts to an invalid and duplicated doctype, resulting in an invalid XML:

<?xml version="1.0"?>
<?xml?>
<!-- ... the rest of the XML is valid -->

If i uncheck the "Ignore attributes" checkbox, then at the 2nd step i'll get a JSON that starts with this

{
    "?xml": {
        "@_version": "1.0"
    },

and when you convert it back to XML then you get a valid, but still duplicated doctype, which results in the same error:

<?xml version="1.0"?>
<?xml version="1.0"?>
<!-- ... the rest of the XML is valid -->

Code

The default XML sample that's in the online tool.

Output

expected data

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• [ ] Yes
• [ ] No

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[amitguptagwl]

Online demo was initially designed for just to demo some capabilities of the application to the new users without being aware with configuration. However, there are so many features added afterwards and it's difficult to cover them with simplified features, we may have to provide some option like "Advance usage" when user can type the configuration.

I am currently occupied in other things. So I would welcome PRs for this