Deleted Root script (.gd) used in Advanced Import Settings not cleared

### Tested versions

4.6 beta 3

### System information

Linux Mint

### Issue description

`Root Script` field in Advanced Import Settings (for .glb) is not cleared after .gd file deleted and make Advanced Import Settings dialog unopenable for this file

### Steps to reproduce

1) Create new script, for ex. `props.gd`

2) Use it in any .glb Advanced Import Settings

3) Press Reimport

4) Delete `props.gd`. Dialog show dependence and said it will deleted

Press `Remove`

5) Try to open this `.glb` file

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`Reload Current Project` _sometimes_ fixing it, no idea for what condition
Renaming used `.gd` file works properly without issues

### Minimal reproduction project (MRP)

N/A