This is essentially the same as #10016, but concerning installing rather than uninstalling.

When installing an add-on via the Add-ons Manager, a user is first asked to confirm that they're happy to install the add-on; if choosing "Yes", the Add-ons Manager then prompts that NVDA needs to be restarted before installation can complete. If a user chooses to install an add-on but then changes their mind before initiating the restart of NVDA (for example they realise that the add-on is Malicious), we could add an "Undo" button to the Add-ons Manager to ensure that the chosen add-on would not be installed and activated upon the next NVDA restart.