Catch No Database Gracefully

[utkonos]

The plugin requires that there be a binary ninja database. If the user attempts to use the plugin on a brand new file that they have not yet saved a database for, the plugin will raise an exception:

[Default] Traceback (most recent call last):
[Default]   File "/Applications/Binary Ninja.app/Contents/MacOS/plugins/../../Resources/python/binaryninja/plugin.py", line 127, in _default_action
[Default]     action(view_obj)
[Default]   File "/Users/username/Library/Application Support/Binary Ninja/repositories/community/plugins/x64dbg_x64dbgbinja/__init__.py", line 40, in export_db
[Default]     outpath = bv.file.database.globals.get('x64dbg_db_save_path', pathlib.Path(bv.file.filename).parent)
[Default] AttributeError: 'NoneType' object has no attribute 'globals'

A check for database existance should be made before the plugin even appears as an option in the various menus. An is_valid callback function that checks for the existance of the database needs to be added and it needs to be called by the register method:
https://api.binary.ninja/binaryninja.plugin-module.html#binaryninja.plugin.PluginCommand:~:text=as%20an%20argument-,is_valid,-(callback)%20%E2%80%93%20optional

This will prevent the plugin's menu entry from appearing at all until a database is available and the plugin would be able to function.