Error occurs when copying (CTRL+C) or saving as mol v3000 'Any Atom' with Isotope (Atomic Mass)

**Steps to Reproduce**
1. Add 'Any Atom' on canvas
2. Add Isotope (Atomic Mass) to 'Any Atom'
3. Select 'Any Atom' and press CTRL+C ( or save as mol v3000)

**Expected behavior**
The atom is copied or saved without errors

**Actual behavior**
An error pop-up appears


Payload:
```
{
"root": {
"nodes": [
{
"$ref": "mol0"
}
],
"connections": [],
"templates": []
},
"mol0": {
"type": "molecule",
"atoms": [
{
"label": "A",
"location": [
2.6750000000000003,
-4.075,
0
],
"isotope": 1
}
]
}
}
```

**Attachments**


**Indigo/Bingo version**
Version 1.16.0-rc.2