fix in cases where a category doesn't already exist

k2t.py

@@ -76,6 +76,10 @@ def t2k(tfile: str, kfile: str) -> None:
     with open(tfile, 'rb') as ff:
         ttree = tomllib.load(ff)
 
+    ttree.setdefault('layers', [])
+    ttree.setdefault('patterns', [])
+    ttree.setdefault('linestyles', [])
+
     entries = [Entry.from_toml(entry)
                for entry in ttree['layers']]
     patterns = [Pattern.from_toml(pat)