Description of problem:
I downgraded a heading from with sub-headings below it.

Version-Release number of selected component:
Zim-0.63-1.fc22

Additional info:
reporter:       libreport-2.6.1
cmdline:        /usr/bin/python /usr/bin/zim --ipc-server-main /tmp/zim-christian/zim-server-socket 30
executable:     /usr/bin/zim
kernel:         4.1.2-200.fc22.x86_64
runlevel:       N 5
type:           Python

Truncated backtrace:
pageview.py:3401:_search_in_widget:AttributeError: 'module' object has no attribute 'tableeditor'

Traceback (most recent call last):
  File "/usr/lib/python2.7/site-packages/zim/plugins/tableofcontents.py", line 373, in on_demote
    self._format(p, newlevel)
  File "/usr/lib/python2.7/site-packages/zim/plugins/tableofcontents.py", line 391, in _format
    if self.select_heading(path):
  File "/usr/lib/python2.7/site-packages/zim/plugins/tableofcontents.py", line 256, in select_heading
    if select_heading(buffer, text):
  File "/usr/lib/python2.7/site-packages/zim/plugins/tableofcontents.py", line 54, in select_heading
    iter = find_heading(buffer, heading)
  File "/usr/lib/python2.7/site-packages/zim/plugins/tableofcontents.py", line 33, in find_heading
    if buffer.finder.find(regex, FIND_REGEX):
  File "/usr/lib/python2.7/site-packages/zim/gui/pageview.py", line 3228, in find
    return self._find_next(iter)
  File "/usr/lib/python2.7/site-packages/zim/gui/pageview.py", line 3264, in _find_next
    for start, end, _ in self._check_range(line, lastline, 1):
  File "/usr/lib/python2.7/site-packages/zim/gui/pageview.py", line 3371, in _check_range
    result = self._search_in_widget(start, step)
  File "/usr/lib/python2.7/site-packages/zim/gui/pageview.py", line 3401, in _search_in_widget
    if isinstance(widgets[0], zim.plugins.tableeditor.TableViewWidget):
AttributeError: 'module' object has no attribute 'tableeditor'

Local variables in innermost frame:
widgets: [<SourceViewWidget object at 0x7f55140430f0 (zim+gui+objectmanager+CustomObjectWidget at 0x2325a60)>]
start: <GtkTextIter at 0x23d35e0>
step: 1
self: <zim.gui.pageview.TextFinder object at 0x7f551520cb50>