Create squashed nifti is not working

This method:

![image](https://github.com/HealthML/visian/assets/77063395/734ad6eb-1da3-416e-9f33-053652e65a6f)

Creates this error during e.g. trying to export a layer:

![image](https://github.com/HealthML/visian/assets/77063395/d7792863-7628-4c69-a4d0-caf05532851d)
