Delete a main dialog will raise 500 server error

[yeze322]

Describe the bug

Problem:
After deleting the main dialog, Composer poped up a 500 error and the project folder was deleted. User won't have a chance to undo this change.

The error always happens even main dialog is not the only existing dialog.

Version

Browser

• Electron distribution
• Chrome
• Safari
• Firefox
• Edge

OS

• macOS
• Windows
• Ubuntu

To Reproduce

Steps to reproduce the behavior:

1. Create a new project
2. Delete the main dialog
3. There will be a 500 server error
4. Reopen this project, it will raise another 500 error
5. The project then disappers

Expected behavior

If deleting main dialog raises errors, we should forbid that behavior.

Screenshots

Additional context

[boydc2014]

@liweitian we should not allow delete the main dialog