Resolved JSON export issues

main.py

@@ -1,4 +1,5 @@
 import os
+import sys
 from flask import Flask
 from src.errors.argument_exception import argument_exception
 from src.export.exporter_factory import exporter_factory

src/convert/model_converter.py

@@ -16,7 +16,7 @@ class model_converter(converter):
             if isinstance(attr, property):
                 v = attr.fget(obj)
                 if issubclass(v.__class__, abstract_reference):
-                    properties[name] = f"{v.id}"
+                    properties[name] = converter_factory.convert(v)
                 else:
                     properties[name] = converter_factory.convert(v)
         return properties

src/models/measurement_unit_model.py

@@ -33,7 +33,7 @@ class measurement_unit_model(abstract_reference):
         """
 
         if self.__base_measurement_unit is None:
-            return self
+            return None
 
         return self.__base_measurement_unit