Bug: Prediction: Error when old & new images coexist

(known error)
To reproduce:
- have a few images already loaded in napari (or left the previous segmentation prediction output image + its raw input images on screen)
- choose "Select an existing model" > "Apply" > Prediction: these images would not be cleared out
- drags & drops a new image to napari to use with prediction
- this error `TypeError: expected str, bytes or os.PathLike object, not NoneType` will pop up even before any interactions with the plugin happens