# Buggy code
def undo(self):
    """Undo removing of a tab or tabs."""
    # Remove unused tab which may be created after the last tab is closed
    last_close = config.val.tabs.last_close
    use_current_tab = False
    if last_close in ["blank", "startpage", "default-page"]:
        only_one_tab_open = self.count() == 1
        no_history = len(self.widget(0).history) == 1
        urls = {
            "blank": QUrl("about:blank"),
            "startpage": config.val.url.start_pages[0],
            "default-page": config.val.url.default_page,
        }
        first_tab_url = self.widget(0).url()
        last_close_urlstr = urls[last_close].toString().rstrip("/")
        first_tab_urlstr = first_tab_url.toString().rstrip("/")
        last_close_url_used = first_tab_urlstr == last_close_urlstr
        use_current_tab = only_one_tab_open and no_history and last_close_url_used

    for entry in reversed(self._undo_stack.pop()):
        if use_current_tab:
            newtab = self.widget(0)
            use_current_tab = False
        else:
            newtab = self.tabopen(background=False, idx=entry.index)

        newtab.history.deserialize(entry.history)
        self.set_tab_pinned(newtab, entry.pinned)


# Traceback (ValueError)
16:39:44 DEBUG    init       app:_init_modules:468 Initializing websettings...
16:39:45 ERROR    misc       crashsignal:exception_hook:216 Uncaught exception
Traceback (most recent call last):
File "/bin/qutebrowser", line 11, in <module>
load_entry_point('qutebrowser==1.1.1', 'gui_scripts', 'qutebrowser')()
File "/usr/lib/python3.6/site-packages/qutebrowser/qutebrowser.py", line 188, in main
return app.run(args)
File "/usr/lib/python3.6/site-packages/qutebrowser/app.py", line 137, in run
init(args, crash_handler)
File "/usr/lib/python3.6/site-packages/qutebrowser/app.py", line 163, in init
_init_modules(args, crash_handler)
File "/usr/lib/python3.6/site-packages/qutebrowser/app.py", line 469, in _init_modules
websettings.init(args)
File "/usr/lib/python3.6/site-packages/qutebrowser/config/websettings.py", line 215, in init
webenginesettings.init(args)
File "/usr/lib/python3.6/site-packages/qutebrowser/browser/webengine/webenginesettings.py", line 297, in init
websettings.init_mappings(MAPPINGS)
File "/usr/lib/python3.6/site-packages/qutebrowser/config/websettings.py", line 198, in init_mappings
mapping.set(value)
File "/usr/lib/python3.6/site-packages/qutebrowser/config/websettings.py", line 72, in set
self._set(value, settings=settings)
File "/usr/lib/python3.6/site-packages/qutebrowser/browser/webengine/webenginesettings.py", line 154, in _set
filenames = [self._find_installed(code) for code in value]
File "/usr/lib/python3.6/site-packages/qutebrowser/browser/webengine/webenginesettings.py", line 154, in <listcomp>
filenames = [self._find_installed(code) for code in value]
File "/usr/lib/python3.6/site-packages/qutebrowser/browser/webengine/webenginesettings.py", line 143, in _find_installed
local_filename = spell.local_filename(code)
File "/usr/lib/python3.6/site-packages/qutebrowser/browser/webengine/spell.py", line 64, in local_filename
all_installed = local_files(code)
File "/usr/lib/python3.6/site-packages/qutebrowser/browser/webengine/spell.py", line 51, in local_files
for matching_dict in sorted(matching_dicts, key=version, reverse=True):
File "/usr/lib/python3.6/site-packages/qutebrowser/browser/webengine/spell.py", line 36, in version
.format(filename))
ValueError: the given dictionary file name is malformed: /usr/share/qt/qtwebengine_dictionaries/en-US.bdic

# Fixed code
def undo(self):
    """Undo removing of a tab or tabs."""
    # Remove unused tab which may be created after the last tab is closed
    last_close = config.val.tabs.last_close
    use_current_tab = False
    if last_close in ["blank", "startpage", "default-page"]:
        only_one_tab_open = self.widget.count() == 1
        no_history = len(self.widget.widget(0).history) == 1
        urls = {
            "blank": QUrl("about:blank"),
            "startpage": config.val.url.start_pages[0],
            "default-page": config.val.url.default_page,
        }
        first_tab_url = self.widget.widget(0).url()
        last_close_urlstr = urls[last_close].toString().rstrip("/")
        first_tab_urlstr = first_tab_url.toString().rstrip("/")
        last_close_url_used = first_tab_urlstr == last_close_urlstr
        use_current_tab = only_one_tab_open and no_history and last_close_url_used

    for entry in reversed(self._undo_stack.pop()):
        if use_current_tab:
            newtab = self.widget.widget(0)
            use_current_tab = False
        else:
            newtab = self.tabopen(background=False, idx=entry.index)

        newtab.history.deserialize(entry.history)
        self.widget.set_tab_pinned(newtab, entry.pinned)