MARelationDescription>>reference half-fix

I don't understand why `self.assertIsInstance(self.inst1.reference, MAStringDescription)`
Same for `self.assertIsInstance(self.inst1.commonClass(), MAPriorityContainer)`

It's a half-fix, because `reference is None` condition never met - supercalss always returns non-nill, see magritte-metamodel/magritte#334

Magritte/MARelationDescriptionTest.py

@@ -6,6 +6,7 @@
 from MANullAccessor_class import MANullAccessor
 from MAStringDescription_class import MAStringDescription
 from MADescription_class import MADescription
+from MAContainer_class import MAContainer
 import MAReferenceDescriptionTest
 
 
@@ -38,10 +39,11 @@ def test_classes(self):
         self.inst1.classes = [int, bool, str]
         self.assertEqual(self.inst1.classes, [int, bool, str])
 
-    def test_commonClass(self):
-        self.assertIsInstance(self.inst1.commonClass(), MAPriorityContainer)
+    def test_commonClass(self):	
+        self.assertIsNone(self.inst1.commonClass())
+
         self.inst1.classes = [MADictAccessor, MAAccessor, MANullAccessor]
         self.assertEqual(self.inst1.commonClass(), MAAccessor)
 
     def test_reference(self):
-        self.assertIsInstance(self.inst1.reference, MAStringDescription)
+        self.assertIsInstance(self.inst1.reference, MAContainer)

Magritte/MARelationDescription_class.py

@@ -29,9 +29,8 @@ def classes(self, aCollection):
 
     def commonClass(self):
         if len(self._classes) == 0:
-            descriptionContainer = MAPriorityContainer()
-            descriptionContainer.label = self.label
-            return descriptionContainer
+            return None
+
         current = next(iter(self.classes))
         for item in self.classes:
             while not issubclass(item, current):
@@ -42,8 +41,15 @@ def commonClass(self):
     def reference(self):
         reference = super().reference
         if reference is None:
-            reference = self.commonClass().magritteTemplate.magritteDescription
-        return reference
+            commonClass = self.commonClass()
+            if commonClass is None:
+                descriptionContainer = self.defaultReference()
+                descriptionContainer.label = self.label
+                return descriptionContainer
+            else:
+                return commonClass.magritteTemplate.magritteDescription
+        else:
+            return reference
 
     @reference.setter
     def reference(self, aDescription):
@@ -52,6 +58,9 @@ def reference(self, aDescription):
     def _reference(self, aDescription):
         super().reference(aDescription)
 
+    @classmethod
+    def defaultReference(cls):
+        return MAPriorityContainer()
 
     def acceptMagritte(self, aVisitor):
         aVisitor.visitRelationDescription(self)