Changing placeholder conversion after importing file with non-string value fails

[Anty0]

Describe the bug
On the import screen, placeholder conversion can be set both before and after importing a file. However, after importing a file that contains a non-string value for any key (e.g., a number in a JSON file), it is no longer possible to change placeholder conversion.

To Reproduce
Steps to reproduce the behavior:

1. Create an empty project
2. Import JSON file

{
  "key": 55
}

4. Try switching placeholder conversion on/off
5. See an error in the server logs