Don't write invalid ref in <str:Categorisation>

Follows fix for #180 in #218.

sdmx/tests/writer/test_writer_xml.py

@@ -318,6 +318,7 @@ def test_data_roundtrip(pytestconfig, specimen, data_id, structure_id, tmp_path)
 @pytest.mark.parametrize(
     "specimen_id, strict",
     [
+        ("BIS/gh-180.xml", False),
         ("ECB/orgscheme.xml", True),
         ("ECB_EXR/1/structure-full.xml", False),
         ("ESTAT/apro_mk_cola-structure.xml", True),

sdmx/writer/xml.py

@@ -480,11 +480,14 @@ def _cl(obj: model.ComponentList, *args):
 
 @writer
 def _cat(obj: model.Categorisation):
-    return maintainable(
-        obj,
-        reference(obj.artefact, tag="str:Source", style="Ref"),
-        reference(obj.category, tag="str:Target", style="Ref"),
-    )
+    elem = maintainable(obj)
+    # If .reader.xml.v21._ref did not resolve a ref while reading (e.g. ref to a non-
+    # standard class like PublicationTable), this may be a dict → don't write. Generates
+    # XSD-invalid SDMX-ML; but occurs only when the input SDMX-ML was invalid anyway.
+    if isinstance(obj.artefact, common.IdentifiableArtefact):
+        elem.append(reference(obj.artefact, tag="str:Source", style="Ref"))
+    elem.append(reference(obj.category, tag="str:Target", style="Ref"))
+    return elem
 
 
 # §10.3: Constraints