Don't ever return None for the type of an Instance (#2568)

Don't ever return None for the str() of an Instance

Author: gvanrossum

mypy/types.py

@@ -1332,7 +1332,10 @@ def visit_deleted_type(self, t: DeletedType) -> str:
             return "<Deleted '{}'>".format(t.source)
 
     def visit_instance(self, t: Instance) -> str:
-        s = t.type.fullname() if t.type is not None else '<?>'
+        if t.type is not None:
+            s = t.type.fullname() or t.type.name() or '<???>'
+        else:
+            s = '<?>'
         if t.erased:
             s += '*'
         if t.args != []: