bugfix: improve disconnect process regarding MediaManager

[pgivel]

Description

This PR fixes up the disconnect process regarding MediaManager. Previously, the manager was not correctly reset and when trying to toggle microphone again the old MicrophoneSender would be reused.

Also a few checks were added for better error handling.

Type of change

• [x] Bug fix (non-breaking change which fixes an issue)
• [ ] New feature (non-breaking change which adds functionality)
• [ ] Breaking change (fix or feature that would cause existing functionality to not work as expected)

Checklist

• [x] I've read & comply with the contributing guidelines
• [x] I have tested my code for new features & regressions on both mobile & desktop devices, using the latest version of major browsers.
• [x] I have made corresponding changes to the documentation (README.md).
• [x] I've checked my modifications for any breaking changes.