Skip blank itemframes.

This may be done improperly. Accessing ref.Item may create an empty compound, and I haven't checked if blank frames have an empty compound for Item or not.

src/mcedit2/rendering/chunkmeshes/entitymesh.py

@@ -339,9 +339,13 @@ class ItemFrameMesh(EntityMeshBase):
             if limitBox and ref.Position not in limitBox:
                 continue
 
+            try:
                 item = ref.Item
                 if item.itemType.internalName != "minecraft:filled_map":
                     continue
+            except KeyError:
+                log.exception("Error while getting ItemFrame item ID in frame at %s", ref.TilePos)
+                continue
 
             mapID = item.Damage