Fix bug when updating add-ons automatically (#18971)
Link to issue number:
Fixes #18965
Summary of the issue:
When NVDA is configured to update add-ons automatically without provide notifications, add-ons aren't updated and an error is produced.
Description of user facing changes:
NVDA can be configured to update add-ons automatically, and this should work as expected.
Description of developer facing changes:
None.
Description of development approach:
Use wx.CallAfter to present a message informing that add-ons are been updated, in the _checkForUpdatableAddonsmethod of the UpdatableAddonsDialog class, just after the condition to check that add-ons should be updated automatically, ensuring that this can be run in the main thread.
Additionally, an error has been discovered and fixed in addonHandler, when installing an add-on which shows a message before installation.
