bug(web): Error occurs when null value given for saving directory

[sevilayerkan]

Describe the bug
Error occurs when null value given for saving directory

To Reproduce
Steps to reproduce the behavior:

1. Go to mainpage in web app
2. Give null value to directory
3. See error

Expected behavior
We should implement error handling by adding a null check to the backend function. Simultaneously, we need to display a warning message to the user. As a preventive measure, we can redirect null directories to the root location.

Screenshots
To be added later

Desktop:

• OS: Windows 10
• Browser: Chrome 116.0.5845.111
• Environment : GitHub Codespaces

[sevilayerkan]

Can be evaluated with #17 task.