FIX: is_reference when None is passed

src/caosdb/common/datatype.py

@@ -66,6 +66,10 @@ def is_reference(datatype):
     RecordTypes
     """
 
+    if datatype is None:
+        raise ValueError("Cannot decide whether datatype is reference if None"
+                         " is supplied")
+
     if datatype in [DOUBLE, BOOLEAN, INTEGER, TEXT, DATETIME]:
         return False
     elif is_list_datatype(datatype):