[Bug] Engine crash when trying to open invalid addon path

### Tested versions

Reproducible in 4.2.1 stable

### System information

Godot v4.2.1.stable - Windows 10.0.19044 - Vulkan (Mobile)

### Issue description

Summary:
I've installed an addon from the asset store. Later on, i decided to change it's location to another folder:
`addons/addon_name` -> `data/addons/addon_name`
Then i tried to change an addon path in project settings (it contained old path)
I clicked on it and engine crashed

### Steps to reproduce

Reproducing:
1. Install any addon
2. Change it's location via external file explorer (not Godot itself)
3. Open project settings
4. Click on addon path

### Minimal reproduction project (MRP)

[mrp_is_not_needed_in_this_case.zip](https://github.com/godotengine/godot/files/14154534/mrp_is_not_needed_in_this_case.zip)

Reproduce the crash by moving an installed addon outside Godot, then opening Project Settings and selecting the old addon path. Trace the project-settings addon-path handling and verify that selecting an invalid path no longer crashes the engine; add a regression test if the existing test structure supports this case.

Written by the indexing model from the issue text.