Error on export (Blender 2.90.1, Windows 10)

Hello,

I'm trying to export using the plugin, but it fails with this issue:
```jsx
Traceback (most recent call last):
File "C:\Users\kevin\AppData\Roaming\Blender Foundation\Blender\2.90\scripts\addons\babylon_js\json_exporter.py", line 47, in execute
if self.settings.writeCsvFile:
AttributeError: 'NoneType' object has no attribute 'writeCsvFile'

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
File "C:\Users\kevin\AppData\Roaming\Blender Foundation\Blender\2.90\scripts\addons\babylon_js\__init__.py", line 65, in execute
exporter.execute(context, self.filepath)
File "C:\Users\kevin\AppData\Roaming\Blender Foundation\Blender\2.90\scripts\addons\babylon_js\json_exporter.py", line 170, in execute
if self.settings.writeCsvFile: stats_handler.close()
AttributeError: 'NoneType' object has no attribute 'writeCsvFile'

location: :-1
```

My setup:

- Windows 10
- Blender 2.90.1

Start with the execute methods in json_exporter.py, especially lines 47 and 170 from the traceback, then inspect the call from __init__.py line 65. Reproduce the export with Blender 2.90.1 on Windows 10; done means the export completes without the reported NoneType writeCsvFile errors.

Written by the indexing model from the issue text.