# Buggy code
def visit_collection(self, node: AnyCollection) -> None:
    """Checks how collection items indentation."""
    elements = node.keys if isinstance(node, ast.Dict) else node.elts
    self._check_indentation(node, elements, extra_lines=1)
    self.generic_visit(node)


# Traceback (AttributeError)
multiprocessing.pool.RemoteTraceback:
"""
Traceback (most recent call last):
File "/usr/lib/python3.7/multiprocessing/pool.py", line 121, in worker
result = (True, func(*args, **kwds))
File "/home/developer2/.local/share/virtualenvs/backend-0eAQt0K2/lib/python3.7/site-packages/flake8/checker.py", line 682, in _run_checks
return checker.run_checks()
File "/home/developer2/.local/share/virtualenvs/backend-0eAQt0K2/lib/python3.7/site-packages/flake8/checker.py", line 612, in run_checks
self.run_ast_checks()
File "/home/developer2/.local/share/virtualenvs/backend-0eAQt0K2/lib/python3.7/site-packages/flake8/checker.py", line 520, in run_ast_checks
for (line_number, offset, text, check) in runner:
File "/home/developer2/.local/share/virtualenvs/backend-0eAQt0K2/lib/python3.7/site-packages/wemake_python_styleguide/checker.py", line 178, in run
yield from self._run_checks(self.visitors)
File "/home/developer2/.local/share/virtualenvs/backend-0eAQt0K2/lib/python3.7/site-packages/wemake_python_styleguide/checker.py", line 166, in _run_checks
visitor.run()
File "/home/developer2/.local/share/virtualenvs/backend-0eAQt0K2/lib/python3.7/site-packages/wemake_python_styleguide/visitors/base.py", line 140, in run
self.visit(self.tree)
File "/usr/lib/python3.7/ast.py", line 262, in visit
return visitor(node)
File "/usr/lib/python3.7/ast.py", line 270, in generic_visit
self.visit(item)
File "/usr/lib/python3.7/ast.py", line 262, in visit
return visitor(node)
File "/home/developer2/.local/share/virtualenvs/backend-0eAQt0K2/lib/python3.7/site-packages/wemake_python_styleguide/visitors/ast/statements.py", line 237, in visit_ClassDef
self.generic_visit(node)
File "/usr/lib/python3.7/ast.py", line 270, in generic_visit
self.visit(item)
File "/usr/lib/python3.7/ast.py", line 262, in visit
return visitor(node)
File "/home/developer2/.local/share/virtualenvs/backend-0eAQt0K2/lib/python3.7/site-packages/wemake_python_styleguide/visitors/ast/statements.py", line 231, in visit_any_function
self.generic_visit(node)
File "/usr/lib/python3.7/ast.py", line 270, in generic_visit
self.visit(item)
File "/usr/lib/python3.7/ast.py", line 262, in visit
return visitor(node)
File "/usr/lib/python3.7/ast.py", line 272, in generic_visit
self.visit(value)
File "/usr/lib/python3.7/ast.py", line 262, in visit
return visitor(node)
File "/home/developer2/.local/share/virtualenvs/backend-0eAQt0K2/lib/python3.7/site-packages/wemake_python_styleguide/visitors/ast/statements.py", line 219, in visit_collection
self._check_indentation(node, elements, extra_lines=1)
File "/home/developer2/.local/share/virtualenvs/backend-0eAQt0K2/lib/python3.7/site-packages/wemake_python_styleguide/visitors/ast/statements.py", line 206, in _check_indentation
extra_lines,
File "/home/developer2/.local/share/virtualenvs/backend-0eAQt0K2/lib/python3.7/site-packages/wemake_python_styleguide/visitors/ast/statements.py", line 171, in _check_first_element
if statement.lineno == node.lineno and not extra_lines:
AttributeError: 'NoneType' object has no attribute 'lineno'
"""

The above exception was the direct cause of the following exception:

Traceback (most recent call last):
File "/home/developer2/.local/share/virtualenvs/backend-0eAQt0K2/bin/flake8", line 11, in <module>
sys.exit(main())
File "/home/developer2/.local/share/virtualenvs/backend-0eAQt0K2/lib/python3.7/site-packages/flake8/main/cli.py", line 16, in main
app.run(argv)
File "/home/developer2/.local/share/virtualenvs/backend-0eAQt0K2/lib/python3.7/site-packages/flake8/main/application.py", line 412, in run
self._run(argv)
File "/home/developer2/.local/share/virtualenvs/backend-0eAQt0K2/lib/python3.7/site-packages/flake8/main/application.py", line 400, in _run
self.run_checks()
File "/home/developer2/.local/share/virtualenvs/backend-0eAQt0K2/lib/python3.7/site-packages/flake8/main/application.py", line 318, in run_checks
self.file_checker_manager.run()
File "/home/developer2/.local/share/virtualenvs/backend-0eAQt0K2/lib/python3.7/site-packages/flake8_per_file_ignores.py", line 33, in run
orig_run(self)
File "/home/developer2/.local/share/virtualenvs/backend-0eAQt0K2/lib/python3.7/site-packages/flake8/checker.py", line 338, in run
self.run_parallel()
File "/home/developer2/.local/share/virtualenvs/backend-0eAQt0K2/lib/python3.7/site-packages/flake8/checker.py", line 302, in run_parallel
for ret in pool_map:
File "/usr/lib/python3.7/multiprocessing/pool.py", line 774, in next
raise value
AttributeError: 'NoneType' object has no attribute 'lineno'

# Fixed code
def visit_collection(self, node: AnyCollection) -> None:
    """Checks how collection items indentation."""
    if isinstance(node, ast.Dict):
        elements = normalize_dict_elements(node)
    else:
        elements = node.elts
    self._check_indentation(node, elements, extra_lines=1)
    self.generic_visit(node)