BUG: Pasting a component in root body layer throws TypeError

GrapesJS version

• I confirm to use the latest version of GrapesJS

What browser are you using?

Chrome v89

Reproducible demo link

https://grapesjs.com/demo.html

Describe the bug

How to reproduce the bug?

1. In the official demo, navigate to the layer manager and copy any layer with cmd+c
2. Click the root "Body" layer and paste with cmd+p
3. Navigate to the browser console and see the error

What is the expected behavior? Pasting in the body layer probably shouldn't be allowed, but it shouldn't throw an error.

What is the current behavior? Pasting in the root Body layer throws this exception:

Uncaught TypeError: Cannot read property 'indexOf' of undefined
    at PasteComponent.js:13
    at Array.forEach (<anonymous>)
    at run (PasteComponent.js:10)
    at r.callRun (CommandAbstract.js:90)
    at Object.runCommand (index.js:388)
    at Object.method (index.js:158)
    at w (keymaster.js:143)
    at HTMLDocument.<anonymous> (keymaster.js:330)

The error is coming from the paste commend; comp.collection is undefined.

I don't know this codebase well enough to know if .collection on this component should return children. We were able to fix this temporarily by overriding the paste command and adding the following guard:

// ...
const coll = comp.collection;
if (!coll) {
    return;
}
const at = coll.indexOf(comp) + 1;
// ...

If this is a fine solution, let me know and I'll happily open a PR.

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