# Buggy code
def analyze_property_with_multi_part_definition(self, defn: OverloadedFuncDef) -> None:
    """Analyze a property defined using multiple methods (e.g., using @x.setter).

    Assume that the first method (@property) has already been analyzed.
    """
    defn.is_property = True
    items = defn.items
    first_item = cast(Decorator, defn.items[0])
    for item in items[1:]:
        if isinstance(item, Decorator) and len(item.decorators) == 1:
            node = item.decorators[0]
            if isinstance(node, MemberExpr):
                if node.name == "setter":
                    # The first item represents the entire property.
                    first_item.var.is_settable_property = True
                    # Get abstractness from the original definition.
                    item.func.is_abstract = first_item.func.is_abstract
            item.func.accept(self)
        else:
            self.fail("Decorated property not supported", item)


# Traceback (AttributeError)
Traceback (most recent call last):
File ".../bin/mypy", line 6, in <module>
main(__file__)
File ".../lib/python3.5/site-packages/mypy/main.py", line 46, in main
res = type_check_only(sources, bin_dir, options)
File ".../lib/python3.5/site-packages/mypy/main.py", line 93, in type_check_only
options=options)
File ".../lib/python3.5/site-packages/mypy/build.py", line 188, in build
graph = dispatch(sources, manager)
File ".../lib/python3.5/site-packages/mypy/build.py", line 1595, in dispatch
process_graph(graph, manager)
File ".../lib/python3.5/site-packages/mypy/build.py", line 1838, in process_graph
process_stale_scc(graph, scc, manager)
File ".../lib/python3.5/site-packages/mypy/build.py", line 1946, in process_stale_scc
graph[id].write_cache()
File ".../lib/python3.5/site-packages/mypy/build.py", line 1576, in write_cache
self.manager)
File ".../lib/python3.5/site-packages/mypy/build.py", line 875, in write_cache
data = tree.serialize()
File ".../lib/python3.5/site-packages/mypy/nodes.py", line 284, in serialize
'names': self.names.serialize(self._fullname),
File ".../lib/python3.5/site-packages/mypy/nodes.py", line 2377, in serialize
data[key] = value.serialize(fullname, key)
File ".../lib/python3.5/site-packages/mypy/nodes.py", line 2325, in serialize
data['node'] = self.node.serialize()
File ".../lib/python3.5/site-packages/mypy/nodes.py", line 2176, in serialize
'names': self.names.serialize(self.fullname()),
File ".../lib/python3.5/site-packages/mypy/nodes.py", line 2377, in serialize
data[key] = value.serialize(fullname, key)
File ".../lib/python3.5/site-packages/mypy/nodes.py", line 2325, in serialize
data['node'] = self.node.serialize()
File ".../lib/python3.5/site-packages/mypy/nodes.py", line 414, in serialize
'items': [i.serialize() for i in self.items],
File ".../lib/python3.5/site-packages/mypy/nodes.py", line 414, in <listcomp>
'items': [i.serialize() for i in self.items],
File ".../lib/python3.5/site-packages/mypy/nodes.py", line 636, in serialize
'func': self.func.serialize(),
File ".../lib/python3.5/site-packages/mypy/nodes.py", line 579, in serialize
'type': None if self.type is None else self.type.serialize(),
File ".../lib/python3.5/site-packages/mypy/types.py", line 766, in serialize
'fallback': self.fallback.serialize(),
AttributeError: 'NoneType' object has no attribute 'serialize'

# Fixed code
def analyze_property_with_multi_part_definition(self, defn: OverloadedFuncDef) -> None:
    """Analyze a property defined using multiple methods (e.g., using @x.setter).

    Assume that the first method (@property) has already been analyzed.
    """
    defn.is_property = True
    items = defn.items
    first_item = cast(Decorator, defn.items[0])
    for item in items[1:]:
        if isinstance(item, Decorator) and len(item.decorators) == 1:
            node = item.decorators[0]
            if isinstance(node, MemberExpr):
                if node.name == "setter":
                    # The first item represents the entire property.
                    first_item.var.is_settable_property = True
                    # Get abstractness from the original definition.
                    item.func.is_abstract = first_item.func.is_abstract
        else:
            self.fail("Decorated property not supported", item)
        if isinstance(item, Decorator):
            item.func.accept(self)