Fix Sequence/Set.prettyPrint crash

When calling `.prettyPrint()` on Sequence/Set components,
make sure the component is a value object (not schema)
because calling `.prettyPrint` on a schema object is
likely to fail.

CHANGES.rst

@@ -4,6 +4,8 @@ Revision 0.4.4, released XX-07-2018
 
 - Fixed native encoder type map to include all ASN.1 types
   rather than just ambiguous ones
+- Fixed crash in `.prettyPrint` of `Sequence` and `Set` occuring
+  at OPTIONAL components
 
 Revision 0.4.3, released 23-05-2018
 -----------------------------------

pyasn1/type/univ.py

@@ -2457,7 +2457,7 @@ def prettyPrint(self, scope=0):
         scope += 1
         representation = self.__class__.__name__ + ':\n'
         for idx, componentValue in enumerate(self._componentValues):
-            if componentValue is not noValue:
+            if componentValue is not noValue and componentValue.isValue:
                 representation += ' ' * scope
                 if self.componentType:
                     representation += self.componentType.getNameByPosition(idx)