Catch Part.DoesNotExist (#5092)

- Can be thrown when importing Part instances
- Fixes https://github.com/inventree/InvenTree/issues/5090

InvenTree/part/models.py

@@ -455,6 +455,7 @@ class Part(InvenTreeBarcodeMixin, InvenTreeNotesMixin, MetadataMixin, MPTTModel)
         """
 
         if self.pk:
+            try:
                 previous = Part.objects.get(pk=self.pk)
 
                 # Image has been changed
@@ -466,6 +467,8 @@ class Part(InvenTreeBarcodeMixin, InvenTreeNotesMixin, MetadataMixin, MPTTModel)
                     if n_refs == 0:
                         logger.info(f"Deleting unused image file '{previous.image}'")
                         previous.image.delete(save=False)
+            except Part.DoesNotExist:
+                pass
 
         self.full_clean()