ENH: add is_reference to db.Property

https://gitlab.com/caosdb/caosdb-pylib/-/merge_requests/63

src/caosdb/common/datatype.py

@@ -45,6 +45,8 @@ def LIST(datatype):
 
 def get_list_datatype(datatype):
     """ returns the datatype of the elements in the list """
+    if not isinstance(datatype, str):
+        return None
     match = re.match("LIST(<|&lt;)(?P<datatype>.*)(>|&gt;)", datatype)
 
     if match is not None:
@@ -66,6 +68,10 @@ 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):