Fix compiler crash.

Cython/Compiler/Nodes.py

@@ -828,7 +828,7 @@ class CSimpleBaseTypeNode(CBaseTypeNode):
                 scope = env
                 for item in self.module_path:
                     entry = scope.lookup(item)
-                    if entry.is_cpp_class:
+                    if entry is not None and entry.is_cpp_class:
                         scope = entry.type.scope
                     else:
                         scope = None